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.

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!)

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!)

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!)

*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!)

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.)

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!)

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...

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