What Determines the Direction of a Red Puck After a Collision?

[ada0289]

A blue puck with a mass of 4.40×10^−2 kg, sliding with a speed of 0.240 m/s on a frictionless, horizontal air table, makes a perfectly elastic, head-on collision with a red puck with mass m, initially at rest. After the collision, the velocity of the blue puck is 7.0×10^−2 m/s in the same direction as its initial velocity.

a) Find the magnitude of the velocity of the red puck after the collision.

b) Find the direction of the velocity of the red puck after the collision. From initial blue puck direction

c) Find the mass of the red puck

Answers:
I was able to get parts a and c but cannot figure out part b.

a) Vred = 0.31m/s

c) Mred = 2.4x10^-2 kg

 

[slider142]

Remember Newton's laws. The red puck was initially at rest and would have remained so unless it was acted upon by a force, which is a vector quantity, and thus has a direction. The puck would then accelerate in the direction of the net force acting on it. How many forces acted on the red puck? What direction was the force in?
You can also analyze this as conservation of momentum, which is also a vector quantity. The direction and magnitude of the net momentum before the collision should be exactly equal to the direction and magnitude of the net momentum after the collision. Since there are only two objects, the net momentum is very easy to visualize.

 

[nlightner]

b) The direction of the velocity of the red puck after the collision would be in the opposite direction of the initial blue puck direction. This is because in a perfectly elastic collision, the total momentum of the system is conserved. Since the blue puck is now moving in the same direction with a higher velocity, the red puck must have lost some of its momentum. Therefore, the red puck would have a velocity in the opposite direction to balance out the momentum.