# newton's third law

• Mar 11th 2008, 07:39 PM
checkmarks
newton's third law
Two hockey players are standing on the ice. One is a Maple Leaf (mass 100kg) and the other is a Canuck (mass 112 kg). If the canuck drives the leaf with a force of 50N

a) what are the action-reaction forces involved in this situation?
b) what is the acceleration of each player?
c) would your answer change if the players drive each other with 50N forces?

physics is definitely not my strongpoint...please help? for part C, i thought their acceleration would both be 0 because their forces cancel out (no net force) but the answer in the textbook said otherwise...
• Mar 12th 2008, 01:19 AM
DivideBy0
Quote:

Originally Posted by checkmarks
Two hockey players are standing on the ice. One is a Maple Leaf (mass 100kg) and the other is a Canuck (mass 112 kg). If the canuck drives the leaf with a force of 50N

a) what are the action-reaction forces involved in this situation?
b) what is the acceleration of each player?
c) would your answer change if the players drive each other with 50N forces?

physics is definitely not my strongpoint...please help? for part C, i thought their acceleration would both be 0 because their forces cancel out (no net force) but the answer in the textbook said otherwise...

Hi checkmarks,

a) The Maple Leaf is pushed by a force of 50N by the Canuck, hence, by Newton's Third Law, the Canuck receives 50N of force from the Maple Leaf.

b) Use F = ma for this one:

First, for the Maple Leaf,

$50 = 100 \times a$

$a=0.50 \ \mbox{ms}^{-2}$

For the Canuck, the problem is slightly different. He experiences 50N of force in the opposite direction to his velocity. This means we need to make the force negative, and he will deccelerate.

$-50=112 \times a$

$a=-0.45 \ \mbox{ms}^{-2}$

c) You can try tackling this with intuition. If a car crashes into a stationary car, it will feel less force than if two cars collide head on at speed.

The Maple Leaf feels 50N from the Canuck, but because he also pushes with 50N, by Newton's Third Law, he feels an additional reaction force of 50N, hence he feels a total force of 100N. This is the same for the Canuck.

In this problem, the acceleration of both players is negative, since the force acts in the opposite direction to their velocity. You should be able to work out the new accelerations. (Yes)