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 the conservation of momentum using the two trolleys, students can carry out the following steps:

1. Set Up the Experiment: Place trolley A on the runway and attach a card to it. Position trolley B on the runway but without any moving forces initially.

2. Measure Initial Velocities: Before the collision, measure the velocity of trolley A, which will be moving towards trolley B.

3. Conduct the Collision: Allow trolley A to collide with trolley B and record their velocities immediately after the collision.

4. Calculate Momentum: Calculate the momentum of both trolleys before and after the collision using the formula: momentum = mass × velocity.

5. Analyze Data: Compare the total momentum before the collision with the total momentum after the collision. The results should show that momentum is conserved if mass and velocity are accurately measured.

The runway is sloped to compensate for the effect of friction. This ensures that trolley A can travel at a constant speed without slowing down due to frictional forces acting against it.

To calculate the average force exerted by the ball on the racket, we use the formula:

$F = \frac{mv - mu}{t}$

Where:

• m = 0.075 kg (mass of the ball)
• v = -15 m/s (final velocity, negative as it is in the opposite direction)
• u = 8.2 m/s (initial velocity)
• t = 0.012 s (time of contact)

Substituting the values:

$F = \frac{(0.075) \times (-15) - (0.075) \times (8.2)}{0.012}$

Calculating:

$F = \frac{(-1.125 - 0.615)}{0.012} = \frac{-1.74}{0.012} = -145 N$

Thus, the average force exerted by the ball on the racket is 145 N (in the direction away from the racket).

Newton's Third Law of Motion states that for every action, there is an equal and opposite reaction. In the context of the collision between the racket and the ball, when the racket exerts a force on the ball to hit it back, the ball simultaneously exerts an equal and opposite force back on the racket. This action-reaction pair is what allows the ball to change direction after the collision.