7. (a) Which of these is a unit of momentum? A kg/m/s B kg/m/s C kg/m/s² D kg/m/s² (b) Students investigate conservation of momentum using two identical trolleys - Edexcel - GCSE Physics - Question 7 - 2022 - Paper 1

To investigate momentum conservation, set up the two identical trolleys on a frictionless runway. Ensure the trolleys are released from rest. Attach a card to one trolley and use a light gate to measure the velocity before and after the collision. Record the masses and speeds of both trolleys before and after the collision. Use the principle of conservation of momentum, which states that the total momentum before the collision equals the total momentum after the collision.

The slope of the runway helps to compensate for the effects of friction, allowing trolley A to travel at a constant speed without slowing down.

First, we need to find the change in momentum: Initial momentum (mu) = mass × initial velocity = 0.075 kg × 8.2 m/s = 0.615 kg m/s Final momentum (mv) = mass × final velocity = 0.075 kg × (-15 m/s) = -1.125 kg m/s Change in momentum = mv - mu = -1.125 kg m/s - 0.615 kg m/s = -1.740 kg m/s. Now, convert the time (12 ms) to seconds: t = 12 ms = 0.012 s. Using the formula: F = (change in momentum) / t = -1.740 kg m/s / 0.012 s = -145.0 N. The average force exerted by the ball on the racket is approximately 145 N in the direction opposite to the ball's incoming velocity.

Newton's Third Law of Motion states that for every action, there is an equal and opposite reaction. When the ball strikes the racket, it exerts a force on the racket, causing it to change direction and speed. Simultaneously, the racket applies an equal and opposite force on the ball, which is what causes the ball to rebound away from the racket.