Presentation: work and energy

Tarp-lined ramp leading down to a pond? Check. All-terrain vehicle? Check. Pulling a cord attached to a raft and rider? Whee! (Video link: "Human Slingshot Slip and Slide - Vooray.")

In this presentation we will introduce a new connection between forces and motion, in terms of how forces can do work on or against an object in order to speed up or slow down its motion. This is a new approach to connecting forces and motion, compared to the previous discussion in this course of using Newton's laws to relate how forces on an object result in a net force that may or many not change its motion.

First, defining the "amount of motion" of an object in terms of its translational kinetic energy, and then expressing how a force may do work on or against the motion of an object.

Translational kinetic energy KE_tr is the energy of motion. (We'll deal with rotational kinetic energy KE_rot later on.)

Translational kinetic energy KE_tr depends on the mass m and the square of the speed v of the object, and the resulting units of kg·m²/s² are also expressed as joules. A stationary object has no translational kinetic energy, and the faster an object moves, the more translational kinetic energy it has.

Instead of finding out how much translational kinetic energy KE_tr an object has, often we are more concerned with its initial-to-final change ∆KE_tr, which is the final amount of translational kinetic energy minus the initial amount of translational kinetic energy. Notice how the common factors of (1/2) and mass m (which is presumed to be constant) are pulled out, leaving a "difference of squares" for the final and initial speeds in the parenthesis.

In order to change the translational kinetic energy of an object (speeding it up or slowing it down), a force (such as that exerted by the horses) must do a certain amount of work W either on, or against the motion of the object (here the loaded sledge the horses are pulling).

Work is accomplished by exerting a force on an object, but in such a way that the object moves through a displacement s (note the unusual use of "s" for a generic ∆x, ∆y, or other direction displacement!), and the "tail-to-tail" angle θ between the force and displacement vectors (when their "bases" are drawn touching together) is anything besides 90°. The units of work are given in N·m, or yet again, joules, so keep in mind that work can be done on or against any mechanical energy form or forms.

Second, let's now explicitly make the connection between the work done by a force acting on or against the motion of an object, and the resulting changes in translational kinetic energy of the object.

This is an incomplete form of the total energy conservation equation we will utilize later, but this "work-energy theorem" shows how the transfer of energy (as work is done by a force acting on or against the motion of the object) causes a corresponding change in the translational kinetic energy of the object.

If the force does work on the object (by being exerted along the direction of its motion), then the work will have a positive sign, and the translational kinetic energy of the object will increase, making the sign of the ∆KE_tr term positive. Note for this case how the left- and right-hand sides of this equation must have the same positive sign.

If the force does work against the object (by being exerted opposite to the direction of its motion), then the work will have a negative sign, and the translational kinetic energy of the object will decrease, making the sign of the ∆KE_tr term negative. Note for this case how the left- and right-hand sides of this equation must have the same negative sign.

Squirrel. Catapult! Squirrel catapult! (Note the person behind the sliding glass door, cutting the release cord with a scissors to launch the squirrel.) (Video link: "squirrelcatapult.gif.")

To keep things simple, let's consider a strictly horizontal version of this contraption, which would make the work done by any vertical forces (such as the weight force of Earth on the squirrel) zero, as the angle between these vertical forces and the horizontal direction of motion is 90°.

The bungee cord (and basket) exerts a force on the squirrel directed to the right, along the direction of motion, so the bungee cord does work on the squirrel. As a result, the squirrel picks up speed (starting from rest), and since translational kinetic energy depends on the square of the speed, since speed increases, then the squirrel's translational kinetic energy increases.)

In the work-energy theorem equation:

W = ∆KE_tr,

the work will have a positive sign (as work was done on the squirrel by the bungee cords), causing the squirrel's translational kinetic energy to increase (and also have a positive sign), so the +/– signs of both the left-hand side and the right-hand side of the equation are consistent with each other:

(+) = (+).

(If you were to calculate the numerical values for the work done (in J) and the resulting numerical value for the increase in translational kinetic energy (in J), then they would have to be equal to each other (along with having the same sign).

For the catapulted squirrel, the bungee cord force does work __________ the squirrel, which __________ the squirrel's translational kinetic energy.)
(A) on; increases.
(B) against; decreases.
(C) (Unsure/lost/guessing/help!)

Note as this car stops, the brakes glow from the heat generated! (Video link: "McLaren SLR review - Top Gear - BBC.")

For the braking car, the brakes do work __________ the car, which __________ the car's translational kinetic energy.
(A) on; increases.
(B) against; decreases.
(C) (Unsure/lost/guessing/help!)

Don't blink, or you'll miss Mrs. P-dog in action! (Video link: "110530-1230869-excerpt.")

For Mrs. P-dog being catapulted upwards, the bungee cords do work __________ Mrs. P-dog, while the weight force does work __________ Mrs. P-dog.
(A) on; on.
(B) on; against.
(C) against; on.
(D) against; against.
(E) (Unsure/lost/guessing/help!)

For Mrs. P-dog's translational kinetic energy to be increased while being catapulted upwards, the amount of work from the bungee cords must be __________ the amount of work from the weight force.
(A) less than.
(B) the same as.
(C) greater than.
(D) (Not enough information is given.)
(E) (Unsure/lost/guessing/help!)

Presentation: energy conservation

Bike, meet hill. Hill, meet bike lift. Bike lift. (Video link: "Bicycle Lift ‘Steep is nothing’ -Panasonic ecoideasnet.")

In this presentation we will complete our introduction to the most common mechanical forms of energy that can be readily transferred amongst each other, along with how this mechanical energy can be irreversibly lost, or irreversibly replenished in the framework of energy conservation.

First, a discussion on forces such as the gravitational weight force, and the force of elastic materials and springs, and how the work done by these forces can be stored and later retrieved without loss. Thus these forces are said to be "conservative," as opposed to non-conservative forces that do work that irreversibly loses or gains energy.

The weight force of Earth on an object is a conservative force, as when this gravitational force does work against an object, removing its energy, it is able to store this energy, and return it to the object by later doing work on the object. Because of this, instead of explicitly calculating the work done against or an object by the weight force during these storage/retrieval processes, it is much more convenient to talk about increases or decreases in this gravitational "potential" energy.

Gravitational potential energy depends on the mass m of the object, the gravitational constant g, and the elevation y of that object. The resulting units of kg·m²/s² are equivalent to joules. An object on the ground has zero gravitational potential energy, and the higher elevation it has, the more gravitational potential energy it has.

Instead of finding out how much gravitational potential energy PE_grav an object has, often we are more concerned with its initial-to-final change ∆PE_grav, which is the final amount of gravitational potential energy minus the initial amount of gravitational potential energy. Notice how the common factors of m and g (which are presumed to be constant) are pulled out, leaving a "difference of elevations" for the final and initial values of y in the parenthesis.

The elastic force of "stretchy" material such as rubber tubing (as in this slingshot), or springs is another conservative force, as when this elastic force does work against an object, removing its energy, it is able to store this energy, and return it to the object by later doing work on the object. Because of this, instead of explicitly calculating the work done against or an object by an elastic force during these storage/retrieval processes, it is much more convenient to talk about increases or decreases in this elastic "potential" energy.

Elastic potential energy depends on the elastic/spring constant k (which is a measure of how difficult it is to stretch/compress) and the distance x that the elastic/spring is stretched/compressed from its relaxed equilibrium (where x is set to be zero). The resulting units of N·m are equivalent to joules. A relaxed elastic/spring contains zero elastic potential energy, and the further it is stretched or compressed from its relaxed state, the more elastic potential energy it has.

Instead of finding out how much elastic potential energy PE_elas an object has, often we are more concerned with its initial-to-final change ∆PE_elas, which is the final amount of elastic potential energy minus the initial amount of elastic potential energy. Notice how the common factors of (1/2) and elastic/spring constant k (which is presumed to be constant) are pulled out, leaving a "difference of squares" for the final and initial values of x in the parenthesis.

Keep in mind that gravitational potential energy and elastic potential energy involve the conservative forces of weight and of elastic materials, which ideally work against or on objects to receive, store, and return energy without loss. Combined with translational kinetic energy KE_tr, PE_grav and PE_elas are the useful "mechanical" forms of energy that can be readily transferred between each form for current or later use.

In contrast, when non-conservative forces are exerted against the motion of objects, energy is lost, irreversibly converted to non-mechanical forms; or when non-conservative forces work along the motion of objects, energy is gained, but this is from the irreversible conversion of non-mechanical forms.

Here we have many examples of non-conservative forces acting on the cat. As it slides across the floor, friction and drag act against the direction of motion of the cat, removing its translational kinetic energy, and it eventually slows to a stop. This energy is irreversibly lost, dissipated to heating up the cat, contact surfaces, and stirring up the air in the room, and cannot be recovered.

Also the people at either end of the kitchen push on the cat along its direction of motion, adding to its translational kinetic energy. This energy is irreversibly converted from the biochemical reactions in the people, and cannot be recovered.

Because these non-conservative forces involve irreversible transfers to/from mechanical energy forms, we must explicitly account for the non-conservative work done by them in energy conservation accounting.

Second, accounting for the transfers between different mechanical energy forms, and also for the contribution to or taking from these energies by non-conservative work.

In contrast to the "incomplete" work-energy theorem, this is the complete form of total energy conservation equation, which shows how the transfer of energy (as work is done by non-conservative forces acting on or against the motion of the object) causes a corresponding change in the translational kinetic energy of the object, along with the potential gravitational energy and the potential elastic energy. Any or all of these energy forms on the right-hand side of the equation can increase or decrease, but together all of their changes must add up to the corresponding non-conservative work term on the left-hand side of this equation.

Note that in the idealized case that there are no non-conservative forces (such as friction, drag, or from external sources), then the left-hand size of this equation would be zero. Then the individual energy terms on the right-hand side of this equation can then trade and balance amongst themselves, instead of with the outside world.

So now let's see how this transfer/balance equation can be applied to idealized situations where friction and drag are negligible.

Let's look at the transfer/balance equation terms for a woman launched by this "human water catapult." Her initial state is just after being launched (and moving upwards with a fast speed); and her final state is at her highest height (where she is momentarily stationary, before coming back down). (Video link: "Human Water Catapult - 55 Foot Launch! In 4k (HD).")

From just after being launched, to reaching her highest height, as the woman is moving upwards her speed is decreasing, and since translational kinetic energy depends on the square of her speed, as her speed decreases, her translational kinetic energy decreases. (If you calculated her change in translational kinetic energy you would get a negative value, which is consistent with a decrease.)

From just after being launched, to just before reaching her highest height, as the woman is moving upwards her height is increasing, and since gravitational potential energy depends on height, as her height increases, her gravitational potential energy increases. (If you calculated her change in gravitational potential energy you would get a positive value, which is consistent with an increase.)

Since we are assuming friction and drag are negligible, then there is no non-conservative work done on the woman, so the left-hand side of the transfer/balance equation is zero:

W_tr = ∆KE_tr + ∆PE_grav + ∆PE_elas,

0 = ∆KE_tr + ∆PE_grav + ∆PE_elas,

and since there are no changes in elastic potential energy while the woman is already moving upwards in the air, we are only left with the changes in the translational kinetic energy and gravitational potential energy terms:

0 = ∆KE_tr + ∆PE_grav + 0,

0 = ∆KE_tr + ∆PE_grav,

0 = (+) + (–).

Now we can see that amount that the woman's translational kinetic energy decreases (where ∆KE_tr is negative) is directly related to the amount that the woman's gravitational potential energy increases (where ∆PE_grav is positive), in order to equal the zero on the left-hand side of the equation. So translational kinetic energy "feeds" (or is "transferred to") gravitational potential energy during this process.

Now for this ball bearing being launched by this slingshot, let's have its initial state as when it is at rest and the elastic bands are fully stretched; and the final state is when the elastic bands are slack with the ball bearing at full speed. (Note that if the slingshot is aimed horizontally, then there is no change in height for the ball bearing, so its gravitational potential energy does not change.) During this process, does the ball bearing's translational kinetic energy increase or decrease? Does the elastic potential energy of the slingshot bands increase or decrease?

Assuming that there is no friction/drag (such that we can neglect non-conservative work), is energy being transferred from translational kinetic energy to elastic potential energy, or is it transferred the other way around? (Video link: "The Physics of Slingshots 2 | Smarter Every Day 57.")

Then for this woman bungee jumper, let's have her initial state as when she is at rest (just before beginning to slide) on top of the roof, with a slack bungee cord; and her final state is when the bungee cord is fully stretched as she swings through her lowest point of descent. For this process, does her translational kinetic energy increase or decrease? Does her gravitational potential energy increase or decrease? Does the elastic potential energy of the bungee cord increase or decrease?

Assuming that there is no friction/drag (such that we can neglect non-conservative work), which energy system is "feeding" the others? Which energy system experienced a greater amount of change (increase or decrease): translational kinetic energy, gravitational potential energy or the elastic potential energy? (Video link: "Homemade Bungee jump.")

Physics presentation: impulse and momentum

Whuuuuuuut. (Video link: "bowling strike with a ping pong ball.")

In this presentation we will introduce another new connection between forces and motion, in terms of how the net force can exert an impulse on an object in order to change its momentum. This is yet another new approach to connecting forces and motion, compared to the previous discussion in this course of using Newton's laws to relate how forces on an object result in a net force that may or many not change its motion, and analyzing how forces can do work on or against an object in order to speed up or slow down its motion.

First, defining the momentum of an object, and then expressing how the net force can exert an impulse on this object.

The introduction slide showing a ping-pong ball knocking over all ten bowling pins should seem very strange to you, as the mass of the ping-pong ball is too small to effectively bowl a strike, even if it were traveling with a supersonic speed. In order to fully account for the "knocking-over" strength of a moving object, then, we must include mass as well as its speed (and direction) to define its momentum p.

Momentum p is a vector quantity (so don't forget to draw an arrow over it) whose magnitude depends both on the mass and speed of the object, with the combined units of both mass and speed (kg·m/s).

We also need to introduce the concept of impulse J, which is the product of the net force acting on an object and the duration of time that the net force acted on this object (whether for a brief instant, or for a prolonged period). (Video link: "Teaching Tee Ball Hitting.")

Impulse has the combined units of both force and time (N·s). Here we use the somewhat obscure (but totally legit) "J" symbol for impulse, remembering to draw an arrow over it (as it is a vector quantity). (It turns out that "I" is already reserved for rotational inertia in the next chapter.)

Second, let's now explicitly make the connection between the impulse acting on an object, and the resulting change in the momentum of the object.

This "impulse-momentum theorem" emphasizes how the impulse (exerted by the net force acting over a specific duration of time) causes a corresponding initial-to-final change in the momentum of the object. And vice versa, where the initial-to-final change in the momentum of an object is caused by the impulse on the object.

Let's apply these concepts to several objects that undergo changes in momentum, with an emphasis on the directions (+/– signs) of these quantities, and how they all must be consistent with each other, starting with a golf ball initially at rest, and then has a speed of 97 m/s after being hit by a golf club. (Video link: "The Moment of Impact. An Inside Look at Titleist Golf Ball R&D.")

This golf ball is initially at rest, so its initial momentum p₀ (mass times its initial velocity) is 0.

We'll define the horizontal direction to be positive to the right (and negative to the left). After it is hit by the golf club, its final momentum (mass times its final velocity) p_f points to the right (and will be a positive quantity).

The initial-to-final change in momentum ∆p of the golf ball is given by:

∆p = p_f – p₀,

and since get a positive quantity minus zero, then ∆p must be positive (thus pointing to the right).

Since the impulse "J" on the golf ball causes this initial-to-final change in momentum:

"J" = ∆p,

the impulse must also have the same direction as ∆p, and so it must also point to the right. (Also since the impulse "J" is the net force ΣF on the golf ball times the contact time ∆t, the net force of the golf club on the golf ball is also directed to the right.)

Now let's have you look at the directions involved in the impulse-momentum theorem for this catapult-launched F/A-18E-F Super Hornet, initially at rest, and then has a speed of 74 m/s after being it is catapulted. (Video link: "F/A-18E-F Super Hornet Catapult Launches.")

Super Hornet's initial momentum p₀ direction? (left (–), none (0), or right (+)?)
Super Hornet's final momentum p_f direction?
Direction of Super Hornet's initial-to-final change in momentum ∆p?
Direction of catapult's impulse "J" on the Super Hornet?

Finally, consider the directions involved in the impulse-momentum theorem for this Ford Ranger, hitting a crash barrier with a speed of 11.0 m/s, and then rebounding off the crash barrier with a speed of 2.2 m/s. (Video link: "Crash Test Ford Ranger 2012....")

Ford Ranger's initial momentum p₀ direction? (left (–), none (0), or right (+)?)
Ford Ranger's final momentum p_f direction?
Direction of Ford Ranger's initial-to-final change in momentum ∆p?
      (Hint: watch your signs!)
Direction of crash barrier's impulse "J" on the Ford Ranger?

Astronomy current events question: polar aurorae on Jupiter

Astronomy 210L, fall semester 2017
Cuesta College, San Luis Obispo, CA

Students are assigned to read online articles on current astronomy events, and take a short current events quiz during the first 10 minutes of lab. (This motivates students to show up promptly to lab, as the time cut-off for the quiz is strictly enforced!)

DC Agle, Michael Buckley, Dwayne Brown and Laurie Cantillo, "Jupiter’s Auroras Present a Powerful Mystery" (September 6, 2017)
nasa.gov/feature/jpl/jupiter-s-aurora-presents-a-powerful-mystery

The brightest __________ observed by NASA's Juno spacecraft in Jupiter's upper atmosphere may have a different energy source than similar phenomena on Earth.
(A) lightning flashes.
(B) hurricane spots.
(C) shooting stars.
(D) ultraviolet jet streams.
(E) polar aurora light.

Correct answer: (E)

Student responses
Sections 70178, 70186
(A) : 4 students
(B) : 0 students
(C) : 0 students
(D) : 6 students
(E) : 38 students

Astronomy current events question: pitch-black exoplanet WASP-12b

Astronomy 210L, fall semester 2017
Cuesta College, San Luis Obispo, CA

Students are assigned to read online articles on current astronomy events, and take a short current events quiz during the first 10 minutes of lab. (This motivates students to show up promptly to lab, as the time cut-off for the quiz is strictly enforced!)

Taylor Bell, Nikolay Nikolov, and Lauren Fuge, "Hubble Observes Pitch Black Planet" (September 14, 2017)
spacetelescope.org/news/heic1714/

The Hubble Space Telescope determined that exoplanet WASP-12b appears pitch black, by analyzing:
(A) dark matter ripples.
(B) polar aurora light.
(C) reflected light.
(D) how much heat it emits.
(E) warped spacetime.

Correct answer: (C)

Student responses
Sections 70178, 70186
(A) : 3 students
(B) : 3 students
(C) : 23 students
(D) : 9 students
(E) : 0 students

Astronomy current events question: titanium oxide in exoplanet WASP-19

Astronomy 210L, fall semester 2017
Cuesta College, San Luis Obispo, CA

Students are assigned to read online articles on current astronomy events, and take a short current events quiz during the first 10 minutes of lab. (This motivates students to show up promptly to lab, as the time cut-off for the quiz is strictly enforced!)

Elyar Sedaghati, Henri Boffin, and Richard Hook, "ESO's VLT Makes First Detection of Titanium Oxide in an Exoplanet" (September 13, 2017)
eso.org/public/news/eso1729/

The European Southern Observatory's Very Large Telescope discovered various chemical compounds in exoplanet WASP-19b by analyzing:
(A) its orbiting moons.
(B) its cloud pattern colors.
(C) light passing through its atmosphere.
(D) how much heat it emits.
(E) radio emission from molecules.

Correct answer: (C)

Student responses
Sections 70178, 70186
(A) : 0 students
(B) : 5 students
(C) : 23 students
(D) : 14 students
(E) : 6 students

Online reading assignment: runaway planets, jovian planets, and dwarf planets (oh my!) (SLO campus)

Astronomy 210, fall semester 2017
Cuesta College, San Luis Obispo, CA

Students have a weekly online reading assignment (hosted by SurveyMonkey.com), where they answer questions based on reading their textbook, material covered in previous lectures, opinion questions, and/or asking (anonymous) questions or making (anonymous) comments. Full credit is given for completing the online reading assignment before next week's lecture, regardless if whether their answers are correct/incorrect. Selected results/questions/comments are addressed by the instructor at the start of the following lecture.

The following questions were asked on reading textbook chapters and previewing presentations on runaway planets (Venus and Mars), jovian planets (Jupiter, Saturn, Uranus and Neptune), and the dwarf planets (and the International Astronomy Union classification scheme).


Selected/edited responses are given below.

Describe something you found interesting from the assigned textbook reading or presentation preview, and explain why this was personally interesting for you.

"The atmospheres of other planets, and how they may have been similar to ours at one point in time."

"That Mercury has a metal core. Is it heavy metal core? I can't imagine that a hunk of metal the size of a planet exists."

"The clouds in Venus's atmosphere are transparent radio waves."

"I think that learning more about planets it really cool; it's interesting to see how unique and different every planet is within our solar system; It makes me wonder about what planets may be out there in the universe (that we have yet to discover/can't discover) that may have even more seemingly unique characteristics."

"Venus and Earth can be so similar but so different at the same time."

"The difference between two similar planets (Mars and Earth). I always thought Mars would be more similar to Earth due to scientists saying that it might possibly be habitable."

"Venus and Earth would have the same amount of carbon dioxide in the atmosphere if it weren't for Earth's oceans dissolving it."

"The importance of oceans on Earth and the fact that Venus was too hot to maintain its oceans, while Mars was too cold to maintain its oceans."

"What makes a planet jovian or terrestrial. Before taking this class I assumed that all the planets were significantly different and it is interesting to see the characteristics of the planets and the similarities to Earth."

"How Jupiter has storms that occur on that planet for centuries. It's interesting how our atmosphere starts and ends storms, while on Jupiter, it goes for much longer."

"I previously thought all planets past mars were 'gas giants,' but learning that Uranus and Neptune are actually considered 'Ice Giants' was pretty interesting to me."

"I thought the Cooper Cooler™ effect was pretty cool to see visually, and I can see myself remembering why Uranus is colder more clearly as a result."

"How simple it is to decide whether or not an object in space is classified as a planet, debris, or a moon/satellite. I just thought it'd be more complex."

"I was really interested in learning about mass and its relevancy. It is review because throughout science classes and episodes of Bill Nye the science guy, it's always been clear that matter matters."

"That Neptune is further from the sun, but yet it is more active than Uranus."

"I didn't know that some planets had their own moons."

"The questions determining whether a object should be a moon/satellite, debris, planet, or a dwarf planet. I didn't know there were so many qualifications for a planet to meet."

Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.

"Something I found confusing (carrying over from last class) was deciding which formations on the terrestrial planets were formed either more or less recently. Reading about the period of heavy bombardment and each planet's unique geological history has helped me understand more but I think it may take more studying before I really get it."

"I didn't know I was confused about the comparisons between, Earth, Venus, and Mars until I started taking this quiz. I found myself doing a lot of additional digging through the book and online to answer the questions."

"Some of the greenhouse factors such as mass and outgassing. How does mass effect the atmosphere and such?"

"I'm a little confused as to how active Venus currently is compared to Earth. Maybe I need to take a break and come back to it instead of just rereading it five times, but I couldn't find in the book or presentations (I only looked over the first one a couple times along with the Venus/Mars section of the book since they seemed to be the most likely places for me to actually find the answer) anything that felt like a definitive answer. There was a part that speculated it has enough energy to break through the surface again, but it also seems to have some activity that create coronae. So, I think that would make it more active that we are today, but if we're only counting stuff that breaches the surface than that would be incorrect."

"I was a little confused with the 'frustrated gophers' comparison to Venus. I just need a little clarification."

"Just the reasoning behind why other planets in are solar system didn't have a better chance at sustaining life. Maybe also just out of curiosity what where the chances of earth ending up being able to sustain life."

"Why are Jupiter's belt zones more visible than Saturn's?"

"I think I have a good grasp of the chapters. It was a sad day when Pluto wasn't deemed worthy to be a planet."

"I was a bit confused why Mercury isn't a dwarf planet, as it is probably small enough."

Identify the relative amounts of these characteristics for Venus, compared to Earth. (Only correct responses shown.)

Interior core heat, today: about the same as Earth [38%]
Geologic activity, today: less than Earth [65%]
Volcanic outgassing, up until now: about the same as Earth [44%]
Heat from the sun: more than Earth [91%]
Amount of atmosphere, today: more than Earth [76%]

Identify the relative amounts of these characteristics for Mars, compared to Earth. (Only correct responses shown.)

Interior core heat, today: less than Earth [85%]
Geologic activity, today: less than Earth [88%]
Volcanic outgassing, up until now: less than Earth [85%]
Heat from the sun: less than Earth [88%]
Amount of atmosphere, today: less than Earth [91%]

Which jovian planet has the coolest interior temperatures?

Jupiter (most massive).	* [1]
Saturn (most prominent rings).	[0]
Uranus (least active weather patterns).	*************** [20]
Neptune (farthest from the sun).	********** [10]
(Unsure/guessing/lost/help!)	*** [3]

I believe Pluto should be a planet.

Strongly disagree.	** [2]
Disagree.	******** [8]
Neutral.	************** [14]
Agree.	******* [7]
Strongly Agree.	*** [3]

Briefly explain your answer to the previous question (whether Pluto should be a planet).

"I think Pluto should be considered a planet because it follows the 3 questions that classify an object as a planet."

"I don't have any personal investment in its status. It just hasn't cleared out its orbital region, it's a dwarf planet."

"I think Pluto should be a dwarf planet from answering the IAU questions. It does orbit around the sun at a slow pace and it is a rounded shape. However, I am unsure if Pluto dominates its orbit, to be on the safe side I'm deciding that it's a dwarf planet."

"Pluto is .07 the size of all other planets in its orbit. It is also outside of the Kuiper belt and as far as we know it could just be a very large comet."

"I think that Pluto is close to being a planet, but it is not strong enough to dominate its orbit."

"Pluto is way too small and there isn't enough activity that takes place there."

"No need to change things."

"Since the discovery of more pluto-like planets in the outer rims of our solar system, i feel like Pluto should not be considered a planet. I don't think that pluto meets all necessary requirements to make it be considered a planet."

"It was a planet until they decided it didn't meet new criteria. I can only go off what the chapter on Pluto said."

"Scientifically speaking, size matters. Pluto isn't even the size of our moon. How could it be considered it's own planet when it could orbit our moon?"

"So they can't continue to mine plutonium (from Rick and Morty)."

"Because when people from the future look back they may get confused on why we stopped calling it a planet and they may freak out."

"I feel as if the reasoning is good behind why Pluto is no longer a planet, but I feel as if they should have been able to determine it earlier than 2006. Maybe that wasn't possible though."

"It was already a planet before, but I don't mind."

"Because the IAU says it's not? A dwarf planet still has planet in the title. 'Plutoid'--really? I think the moon should be a planet! I think anything we could possibly land on and terraform to colonize should be considered a planet. Probably watched too many sci-fi shows."

"It shouldn't be a planet because it is too small and is not terrestrial nor jovian. There are many objects like Pluto and that is why all of those should be in their own category."

"I don't really have a strong opinion on it, but I do think that it's alright to me that pluto isn't a planet, but a dwarf planet. It's fairly small and the other dwarft planets seem to be about the same size or even smaller than Pluto."

"Because it was most of my life, and mainly because even though it seems to be randomly floating it is still in orbit for the most part."

"It doesn't really matter to me all that much, it's not like it disappeared."

"Yes, Pluto used to be classified as a planet, but since it's so incredibly tiny it can't even pull other objects into its orbit. But it was able to put itself in a spherical shape which is one of the requirements of being a planet. But I think it being classified as a dwarf planet suits it better. It has its positives and negatives of whether it should be a planet so I can't make up my mind on if it should be or not."

"Pluto was always a planet when I was in school, if it ain't broke don't fix it."

"It's orbital path is different than the other planets, but it is round and also orbits the sun."

"I believe Pluto should be a planet because we grew up thinking it was part of the planet system for a long time. However, this planet has a couple different charactericts compare to others that might make it seem like it's not appropriate to call it a 'planet.'"

"Pluto is considered to be part of the some 100 million objects in the Kuiper belt, considered mere remnants from the formation of the solar system."

"If the third question we ask about a body is whether it dominates orbit, then it must be rejected that it is a planet, because it shares orbit with many similar objects without pulling them in."

"Pluto was classified as a planet for so long, it didn't need to be changed. It may not have the same orbit as the other planets, but it still does orbit the sun. It also may not be as big, but it there is no requirement for a object to be a certain size to be considered a planet."

"It isn't related to jovian or terrestrial planets."

"I think Pluto should be considered a planet, it always has been, and always should be."

"I do think it has mostly planet characteristics, but it is really small (its moon is even almost as big as it) and isn't clear of debris so it should stay a dwarf planet."

"On some level I think Pluto should be considered an honorary planet because it has been included as one in our solar system models for so long. However, since it fails to meet the IAU classification requirements the title 'dwarf planet' should be used for consistency's sake (also 'dwarf planet' is a cute descriptor)."

"I just feel as though Pluto falls so closely into being a planet, that it might as well already be a planet. However, it still does lack some of the official planet criteria. So I think it could go either way."

"I take no personal value from Pluto being a planet or not. I think if Pluto had a really good argument and all it wanted its entire life was to be a planet, then similar to sex changes, it should be able to transition."

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"How theoretically would you try to warm a cold planet like Mars to try to get it to sustain life?" (There are lots of ideas for this, based on either introducing more greenhouse gases, or adding more heat to the atmosphere (mirrors, etc.).)

"What is your take on colonizing Mars, and if it were possible would you be down to move there?" (As much as I like backpacking, I feel that living on Mars would just be way too much of that.)

"Can you go over the characteristics of Venus and Mars in comparison to Earth? (Especially in regards to whether Venus is more/less geologically active than Earth today)"

"Would it be possible to go over greenhouse factors and runaway atmospheres in our next class session?"

"In the jovian planets presentation, I was a little confused on how to tell if a planet is cooler than the other. If a planet has less cloud action or color, is it cooler or warmer?" (Cooler; the core heat drives the circulation of the entire liquid/gas structure of the planet, so less core heat means less circulation, and quieter action (and color, since there is less energy to drive chemical reactions).)

"I remember being little in school and hearing Pluto was no longer a planet and I was sad about it. I don't know why."

"Do you think Pluto should be considered a planet?" (No. My philosophy is that life isn't always fair. Deal with it, Pluto.)

"Where do you go to look at the stars?" (Wherever Slumberjack, the Sleeping Forester can take me and Mrs. P-dog.)

"Can we use any notes on the final exam?" (No. But you will get a study guide on specifically what you need to know.)

Online reading assignment: uniform circular motion

Physics 205A, fall semester 2017
Cuesta College, San Luis Obispo, CA

Students have a bi-weekly online reading assignment (hosted by SurveyMonkey.com), where they answer questions based on reading their textbook, material covered in previous lectures, opinion questions, and/or asking (anonymous) questions or making (anonymous) comments. Full credit is given for completing the online reading assignment before next week's lecture, regardless if whether their answers are correct/incorrect. Selected results/questions/comments are addressed by the instructor at the start of the following lecture.

The following questions were asked on reading textbook chapters and previewing a presentation on uniform circular motion.


Selected/edited responses are given below.

Describe what you understand from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically demonstrate your level of understanding.

"In uniform circular motion, the direction of the net force points inwards. Newton's second law still applies as the direction is continuously changing."

"When an object is being spun in a circular motion, if released it will continue in a straight line."

"How Newton's second law plays a role in uniform circular motion, that the direction is changing. And that the acceleration is always is directed towards the center."

"Circular motion requires Newton's second law of motion. This includes uniform circular motion. The direction is always changing and acceleration always points towards the center."

"Newton's second law applies to uniform circular motion. When applied to motion with constant speed the direction of the net force always points inward (which is the centripetal direction)."

"When an object is turning or going in a circle, it experiences acceleration, therefore Newton's second law applies. There is also a net force towards the center of the circle, perpendicular to the instantaneous velocity of the object, which explains why it appears to turn towards in its direction."

"The centripetal force is not actually a 'force' but it is the net sum of all the forces acting on an object that is undergoing uniform circular motion. The direction of centripetal force always points toward the center of the circle it is moving along."

"Centripetal force is not a new force but instead is the net force pointing towards the center of a circular path. The centripetal force itself can be a rope pulling an object inward or be inside of a turning car pushing someone sideways."

Describe what you found confusing from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically identify the concept(s) that you do not understand.

"Vertical circular motion."

"Acceleration is towards the center?"

"When an object is in constant speed going around a circle, why is that according to the book we use Newton's second law?"

"I am unsure how to apply these GIF animations to centripetal force or how to use it."

"This chapter isn't necessarily confusing but I think I will need help when applying these concepts to problems, and also I will want to see the answers to the examples below because I could be totally off on that as well."

"I understood this chapter, I think."

For the "drifting" car (skidding around a circular track at constant speed), Newton's __________ law applies to its motion, and the forces acting on it add up to a net force that:

first; is zero.	*** [3]
second; points to the left.	*************** [15]
second; points to the right.	************************ [24]
(Unsure/lost/guessing/help!)	** [2]

At the moment when the woman is at the bottom of her swinging trajectory (when the rope is vertical), Newton's __________ law applies to her motion, and the forces acting on her add up to a net force that:

first; is zero.	********** [10]
second; points upwards.	*********************** [23]
second; points downwards.	********* [9]
(Unsure/lost/guessing/help!)	** [2]

At the moment when the motor scooter is on the left side of the screen (traveling out at you), Newton's __________ law applies to its motion, and the forces acting on it add up to a net force that:

first; is zero.	******* [7]
second; points to the left.	********* [9]
second; points to the right.	*********************** [23]
(Unsure/lost/guessing/help!)	***** [5]

At the moment when the car is at the very top of the loop-the-loop, Newton's __________ law applies to its motion, and the forces acting on it add up to a net force that:

first; is zero.	****** [6]
second; points upwards.	******* [7]
second; points downwards.	***************************** [29]
(Unsure/lost/guessing/help!)	** [2]

At the moment when a person is at the right edge of the screen (traveling out at you), Newton's __________ law applies to his/her motion, and the forces acting on him/her add up to a net force that:

first; is zero.	******** [8]
second; points to the left.	******************** [20]
second; points to the right.	********* [9]
(Unsure/lost/guessing/help!)	******* [7]

At the moment when the car is at the very top of its mid-air trajectory, Newton's __________ law applies to its motion, and the forces acting on it add up to a net force that:

first; is zero.	******** [8]
second; points upwards.	*** [3]
second; points downwards.	*************************** [27]
(Unsure/lost/guessing/help!)	****** [6]

At the moment when the skateboarder is at the very top of his mid-air trajectory, Newton's __________ law applies to his motion, and the forces acting on him add up to a net force that:

first; is zero.	**** [4]
second; points upwards.	******** [8]
second; points downwards.	******************************* [31]
(Unsure/lost/guessing/help!)	* [1]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"I think that I am now pretty clear when Newton's first, second or third law applies. It was a bit challenging at first, but after the in-class examples it was much more clear."

"As always, it seems to be the hardest part of all of it is practicing all the concepts and examples in class. It seems like the biggest part is just understanding how to use the concepts to apply them to problem-solving. Just need to practice more."

"Good examples in the reading assignment this time."

"I still can't tell what determines if these situations are a Newton's first law or second law situation."

"I need more explanations, it's hard to determine which Newton's law applies for these situations."

"If possible, could we please review the above GIF animations very briefly for some clarifications? I am a little confused but I think I'll get it quickly with a little more instruction. Thanks."

"I feel like I got all of these wrong."

"I am confused concerning the difference between centripetal forces and centrifugal forces." ("Centripetal" and "centrifugal" merely refer to directions that point in towards or out away from the center of a circle ("center-seeking" and "center-fleeing"). Strictly speaking, they're just direction labels, like "up" and "down." The textbook is confusing in that it refers to "the uniform circular motion net force" as "the centripetal force." ¯\_(ツ)_/¯)

"I don't get the process for calculating centripetal force on an object going around a banked turn." (Don't worry about that case of uniform circular motion for this class; we're going to busy enough as it is with horizontal and vertical objects undergoing "flat" uniform circular motion.)

"It is difficult to keep the right pace in this class, I am half a week behind the schedule, meaning I am usually working on the content of last week when we are starting a new topic. I'm scared I'm going to miss something important so I just go over more stuff than we need since we don't have a lecture. It makes it difficult to catch up. :(" (The amount of material we're covering at this pace can be overwhelming to keep up with, but I'll always emphasize in class what is important, and how you'll be tested on what's important. You're not expected to know everything and calculate everything covered in the textbook! See how you do on the practice quiz in class on Wednesday; this should help you figure out where you are in terms of your studying before next Monday's quiz.)

"Is this why hurricanes are the weakest on the outside and the strongest inside? Also how would this apply to the eye of the hurricane? Also, I don't mind that you don't do lectures in class--please don't start doing more of them because for me the most helpful part are the examples and worksheets, I can always read the book at home." (Short answer: it's complicated. Longer answer: the column of warm air that rises at the center of the hurricane is replaced by cooler air that rushes in towards the center, and this where wind speed inwards and upwards is greatest; but Earth's rotation will "twist" this inflow of cooler air into the characteristic spiral. Then the warm air at the center will spiral upwards as it rises, becoming the "eye" of the hurricane--a gigantic tornado sustained by the continuous inward spiraling flow on cooler air. So basically, thermodynamics, which is still physics.)

"Great job so far! I'm enjoying the class."