Video Transcript
Two balls of equal mass collide head-on, as shown in the diagram. The balls move at speeds big 𝑉 and little 𝑣, respectively, where big 𝑉 is greater
than little 𝑣. Which of the following diagrams correctly represents the reaction forces that would
act when the balls collide, ignoring any objects other than the two balls? Is it diagram (A), (B), (C), (D), or (E)?
In this question, we have a pair of interacting objects, so we know that Newton’s
third law of motion must apply to this situation. Let’s recall that this law states that for a pair of interacting objects A and B, if
object A exerts a force on object B, then B exerts an equal and opposite force on
object A. In other words, we know that for these two balls here, their reaction forces must act
in opposite directions and with equal magnitude.
Now the correct answer must show two forces acting here, one on each ball. So let’s eliminate option (A).
The remaining options all do show two forces pointing in opposite directions. The correct answer option will show the two forces as having equal magnitude, which
is represented by two arrows of the same length. Answer options (B), (C), and (E) show forces with unequal magnitudes. So we know those are wrong.
Thus, we know that option (D) is the correct answer. This diagram correctly represents the reaction forces that would act when the balls
collide, since the forces shown obey Newton’s third law of motion.
