Don't push the referee
An ice hockey player is given a red card by the referee. Out of rage, the player charges towards the referee and gives him a push. Which of the following is/are experienced, equally in magnitude, by both of them?
A.
Force
B.
Change in momentum
C.
Acceleration
D.
Change in kinetic energy
Note:
The referee and the player weigh differently.
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1 solution
First, we should understand that magnitude means the absolute value (distance from zero) of a number: I.e. The absolute value of both 10 and -10 is 10.
Newton's 3rd law states that when two objects interact, they interact with an equal and opposite force - This implies A is true, but then all of the answers contain A. However, we can use this fact later.
F o r c e = M a s s × A c c e l e r a t i o n
If the Player's and the Referee's masses are not equal (while the magnitude of their force is equal), they must have different acceleration magnitudes - This implies C is false, which eliminates 'A, B, C, D' and 'A and C' as potential answers.
Newton's 2nd law states that the rate of change of momentum of a body, is directly proportional to the force applied. As we now know from Newton's 3rd law, the force between the Player and the Referee has equal magnitude. Thus, their change in momentum must also have equal magnitude - This implies that only A is false, which eliminates 'Only A' as a potential answer.
A and B
We could look into whether Kinetic Energy is also valid, but this is moot. The only answer which contains D, that's been offered as a choice, has been proved invalid (because it also contains C).