Question

A 50 kg skater at rest on a frictionless rink throws a 2 kg ball, giving the ball a velocity of 10 m/s.
Which statement describes the skater's subsequent motion?
(1) 0.4 m/s in the same direction as the ball's motion
(2) 0.4 m/s in the opposite direction of the ball's motion
(3) 2 m/s in the same direction as the ball's motion
(4) 2 m/s in the opposite direction of the ball's motion

Answer 1

Explanation:

Given A 50 kg skater at rest on a friction less rink throws a 2 kg ball, giving the ball a velocity of 10 m/s.

Which statement describes the skater's subsequent motion?

• Applying the law of conservation of momentum that is total momentum of a system remains constant when there are no outside forces.
• (m1v1 + m2v2 +…..) initial = (m1v1 + m2v2 + ……) final
• Initial momentum is zero and final momentum of ball is positive. Now final momentum of skater must be negative to balance.
• 50 x 0 + 2 x 0 = 50 v1f + 2 x 10
•    0 = 50 v1f + 20
• So v1f = - 0.4 m / s
• So skater moves with a speed of 0.4 m/s in the opposite direction of the ball.