Three hockey pucks collide on the ice. Hockey puck A hits hockey puck B with a force of 9N to the right. Hockey puck B hits hockey puck C at the same moment and in the same direction with a force of 4 N, not including the force from puck A. What is the resultant force on hockey puck B?

Question

Quizplus · Accepted Answer

In this problem, it is crucial to remember Newton's Third Law which is: The mutual forces of action and reaction between two bodies are equal, opposite and collinear.
Step 1: Analyze the collision between Puck A and Puck B
We know that puck A hits puck B with a force of 9N to the right. 
Step 2: Analyze the collision between Puck B and Puck C
When puck B collides with puck C with the combined force of both puck A (9N to the right) and the force puck B brings (4 N to the right). To determine the net force of an object that has two forces in the same direction, simply add them use the following equation: NetForce = F1 + F2
NetForce = F1 + F2
NetForce = 9 N + 4 N
NetForce = 13 N to the right 
Because there is an equal and opposite force from the collision, Puck B also experiences a force of 13 N from the collision between B and C but to the left. 
Step 3: Combine to get the net force
To determine the net force of an object that has two opposing forces, use the following equation: NetForce = Fleft + (-) Fright.
Force on B from collision A/B: 9 N to the right
Force on B from collision B/C: 13 N to the left
NetForce = Fleft + (-) Fright
NetForce = 13 N + (-) 9 N
NetForce = 4 N = 4 N to the left

Three Hockey Pucks Collide on the Ice