## 20150719

### Physics presentation: forces and motion

Previously we introduced forces: how objects interact with each other, represented by vectors, which we introduced even earlier. Let's tie together forces, motion, and vector addition.

Now we'll introduce Newton's first law and second law, which connect forces with motion. In order to determine whether Newton's first law or second law applies, we can either consider the motion of an object, or the net force on an object.

First we'll analyze the motion of an object, in order to determine which Newton's law applies to it.

This flowchart asks two questions to distinguish between Newton's first law and second law cases.

First, is the speed constant? If not, then Newton's second law applies. But if yes, speed is constant, then you can move on to the second question.

Second, is the direction of motion constant? If not, then Newton's second law applies. But if yes, direction is constant (and speed as well), then Newton's first law applies. Note that the logic to classify a Newton's first law case is considered more restrictive than for a second law case. Let's see how this logic flow works by looking at the motion of different objects.

(Movie link: "MythBusters--Rocket Sled Destruction Angle 2.")
For this rocket sled, Newton's __________ law applies to the motion of this object, and the forces acting on the object add up to a __________ net force.
(A) first; zero.
(B) second; non-zero.
(C) (Unsure/lost/guessing/help!)

(Movie link: "F-35B Vertical Landing Highlights w/ test pilot commentary.")
While the F-35B is descending at a steady rate (before it touches the ground), Newton's __________ law applies to the motion of this object, and the forces acting on the object add up to a __________ net force.
(A) first; zero.
(B) second; non-zero.
(C) (Unsure/lost/guessing/help!)

For this car with a steady speedometer reading (and assuming the shaking is due to the person holding camera, and not from moving the car itself), Newton's __________ law applies to the motion of this object, and the forces acting on the object add up to a __________ net force.
(A) first; zero.
(B) second; non-zero.
(C) (Unsure/lost/guessing/help!)

(Movie link: "Spinning Swing ride at Lightwater Valley Theme Park.")
For a person in the swinging chair ride moving along a circular trajectory at a constant speed, Newton's __________ law applies to the motion of this object, and the forces acting on the object add up to a __________ net force.
(A) first; zero.
(B) second; non-zero.
(C) (Unsure/lost/guessing/help!)

For this car with a zero speedometer reading, Newton's __________ law applies to the motion of this object, and the forces acting on the object add up to a __________ net force.
(A) first; zero.
(B) second; non-zero.
(C) (Unsure/lost/guessing/help!)

Now let's approach Newton's first law and second law by considering the net force on an object.

Note that this net force flowchart is simpler than the motion flowchart--there is only one question that needs to be answered: Is the net force zero? If not, then Newton's second law applies. If yes, the result of adding all forces acting on an object is zero, then Newton's first law applies. Let's see how this logic flow works by looking at the forces acting on different objects.

For only one force, Newton's __________ law applies to the motion of this object, and the forces acting on the object add up to a __________ net force.
(A) first; zero.
(B) second; non-zero.
(C) (Unsure/lost/guessing/help!)

For two (opposite, equal magnitude) forces, Newton's __________ law applies to the motion of this object, and the forces acting on the object add up to a __________ net force.
(A) first; zero.
(B) second; non-zero.
(C) (Unsure/lost/guessing/help!)

(This photo was featured on Gizmodo in 2013!)
For two (opposite, unequal magnitude) forces, Newton's __________ law applies to the motion of this object, and the forces acting on the object add up to a __________ net force.
(A) first; zero.
(B) second; non-zero.
(C) (Unsure/lost/guessing/help!)

For two (diagonal, equal magnitude) forces, Newton's __________ law applies to the motion of this object, and the forces acting on the object add up to a __________ net force.
(A) first; zero.
(B) second; non-zero.
(C) (Unsure/lost/guessing/help!)

(Later, we'll consider cases where there could be three or more forces acting on an object.)

Now that we know which Newton's law applies given the motion of an object, and which Newton's law applies given the net force acting on an object, let's use Newton's laws to relate the motion of an object with the net force.

Newton's second law is the case where the motion of an object (whether speed, direction, or both speed and direction) changes, which means that the acceleration is non-zero. Also the net force for a Newton's second law case must also be non-zero. Since mass is non-zero, then both sides of the equation are non-zero--there must be a net force, and motion must change (speed, direction, or both speed and direction changing).

What effect does mass have in Newton's second law, for accelerating objects? (Movie link: "110725-1240641-r.") The fans attached to each cart provides the same amount of force to propel it from rest.
Assuming that the drag and friction forces on each cart are also the same, the __________ cart has a greater net force exerted on it, such that it has a greater acceleration.
(A) less massive.
(B) more massive.
(C) (There is a tie.)
(D) (Unsure/lost/guessing/help!)

Newton's first law is the case where the motion of an object is constant (both speed and direction constant), which means that the acceleration is zero. Also the net force for a Newton's first law case must be zero. Since mass is non-zero, then both sides of the equation must be zero--if there is no net force, then the motion must be constant (in both speed and direction).

Looking ahead, how many Newton's laws are there? (Three.) But there are only two ways to classify motion--constant, or changing, corresponding to Newtons first law, or second law. Or only two ways to classify net force--zero, or non-zero, corresponding to Newton's first law, or second law.

So if there are only two types of motion, and two types of net forces, what's up with Newton's third law? As it turns out, Newton's third law has nothing to do with motion or net force, but something else entirely, something much more universal and encompassing than considering a particular type of motion or net force...