Video Transcript
Two balls, A and B, are colliding
together as shown in the figure below. If ball A, during the collision,
exerts a force 𝐹 on ball B, then ball B exerts a force of blank on ball A. (A) 𝐹, (B) negative 𝐹, (C) two 𝐹
over five, (D) five 𝐹 over two.
In this question, we are being
asked for the force exerted by ball B on ball A when the two balls collide.
To answer this question, we will
need to recall Newton’s third law of motion. Remember that Newton’s third law
applies whenever there are two objects that interact with each other. In this case, we have two balls
that collide, and this collision constitutes an interaction. Newton’s third law is often
paraphrased as saying that every action has an equal and opposite reaction. What this means is that if we have
two objects and object one exerts a force on object two, then object two also exerts
a force on object one that is equal in magnitude but opposite in direction.
Now, we’re told that ball A exerts
a force 𝐹 on ball B, that is, a force with a magnitude of 𝐹 and that acts in the
positive direction. In the sketch we’re given, the
balls collide, with ball A moving to the right and ball B moving to the left. That means that when they collide,
ball A must exert a force on ball B that acts in the rightward direction. That is, right is being taken as
the positive direction here.
Newton’s third law tells us that
ball B will also exert a force on ball A, with the same magnitude of 𝐹 but pointing
in the opposite direction. This means that the force exerted
by ball B on ball A is a force with a magnitude of 𝐹 that acts in the negative,
leftward direction. This is the same as saying the
force exerted by ball B on ball A is equal to negative 𝐹. Therefore, the correct answer is
option (B), negative 𝐹.
Notice, by the way, that we didn’t
actually need to use the information about the masses of the two balls that was
given to us in the diagram. Newton’s third law of motion does
not depend on the masses of the two objects involved.
