Impulse and force-momentum changeEdexcel International A Level Physics: Subtopic test

1. 1
   A tennis ball of mass 0.057 kg travels horizontally at 25 m s⁻¹ towards a racket. It is in contact with the racket for 5.0 ms and leaves in the opposite direction at 30 m s⁻¹.

   (a)

   The impulse of the racket on the ball is equal to which quantity?

   [1 mark]

   • AThe change in kinetic energy of the ball
   • BThe change in momentum of the ball
   • CThe momentum of the ball before the impact
   • DThe force divided by the contact time

   (b)

   What is the magnitude of the change in momentum of the ball?

   [1 mark]

   • A0.29 kg m s⁻¹
   • B1.4 kg m s⁻¹
   • C1.7 kg m s⁻¹
   • D3.1 kg m s⁻¹

   (c)

   Calculate the average force exerted by the racket on the ball.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In a crash test a dummy of mass 75 kg is travelling at 14 m s⁻¹ in a car that stops suddenly. The seat belt brings the dummy to rest in 0.20 s.

   (a)

   Calculate the average force on the dummy during the stop.

   [1 mark]

   • A2.1 × 10² N
   • B1.1 × 10³ N
   • C5.3 × 10³ N
   • D1.1 × 10⁴ N

   (b)

   An airbag is fitted so that the dummy stops more slowly. Why does this reduce the average force on the dummy?

   [1 mark]

   • AThe same change of momentum happens over a longer time
   • BThe change of momentum is smaller
   • CThe dummy has less kinetic energy
   • DThe impulse is larger

   (c)

   The airbag increases the stopping time to 0.50 s. Calculate the average force on the dummy now.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In Core Practical 9 a student investigates the relationship between the force on a trolley and its change of momentum. The trolley has mass 0.80 kg and is pulled along a track by a falling weight on a string. Light gates record its speed as 0.40 m s⁻¹ at the first gate and 1.20 m s⁻¹ at the second gate, and a data logger records 2.0 s between the gates.

   (a)

   Calculate the change in momentum of the trolley between the gates and the average resultant force on it.

   [3 marks]

   (b)

   Describe how the student should vary the force and use the results to show that, for a fixed time, the change of momentum is proportional to the force. Suggest how she should reduce the effect of friction.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Hailstones of mass 5.0 g fall vertically at 20 m s⁻¹ onto a flat horizontal greenhouse roof of area 12 m². On average 400 hailstones strike the roof each second.

   (a)

   Show that impulse equals change of momentum, starting from Newton's second law in the form F = ma. Hence calculate the average force and the average pressure on the roof if the hailstones stop on impact.

   [6 marks]

   (b)

   The hailstones rebound vertically upwards at 10 m s⁻¹ instead of stopping. Calculate the new average force and pressure on the roof, and explain why the force is greater than when the hailstones stop.

   [6 marks]

   Total for question 4: 12 marks

End of questions