Cambridge International AS and A level Physics
4 This question is about Newton’s third law of motion. a Two bar magnets are placed close to one another with their north poles facing each other. i
State whether the magnets attract or repel each other.
ii What does Newton’s third law tell you about the force each magnet exerts on the other? b If you stand on the floor, two forces act on you: your weight and the upward contact force of the floor. i
Explain why these two forces are not an ‘equal and opposite pair’ in the sense of Newton’s third law.
ii For each of the two forces, state the force that is equal and opposite to it, as described by Newton’s third law. Remember that ‘weight’ is the Earth’s gravitational pull on an object.
Exam-style questions 1 A bullet of mass 25 g is travelling at 450 m s −1 when it strikes the armour plating of a tank. It bounces back along the same path with a speed of 390 m s −1.
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a Calculate the change in momentum of the bullet.
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b The time of impact of the bullet on the tank is 0.040 s. Calculate the average force that acts on the bullet in this time.
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c State whether the impact is elastic or inelastic. Explain your answer.
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d Discuss how the principles of conservation of energy and momentum apply to this collision.
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2 In an experiment to measure the speed of a bee as it flies, a small ball is hung by a thread. When the bee lands on the ball, the ball swings upwards. Scientists record a video of the bee as it lands and analyse the video to determine the height to which the ball swings.
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mass of bee = 0.25 g; mass of ball = 0.45 g a The ball with the bee attached to it is found to rise to a vertical height h of 6.0 cm. Calculate its increase in gravitational potential energy.
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b Determine the speed of the ball+bee when it starts to swing upwards.
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c Determine the momentum of the ball+bee.
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d Determine the speed with which the bee lands on the ball.
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e Suggest the optimum horizontal point of the swing for the bee to take off from the ball. State your criteria for choosing this point and explain your answer.
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