Chapter 6: Momentum
a The spacecraft has a mass of 420 kg. Calculate the change in its momentum as it travels halfway around its orbit. b The force on the spacecraft keeping it in orbit is its weight. Calculate its weight, if the gravitational field strength is 8.9 N kg−1. c Use your answers to parts a and b to calculate the time taken for the spacecraft to complete half an orbit, and hence find its orbital period. 3 Your friend is running down the street and bumps into someone, who falls to the ground. The person complains, “You really hurt me!” Your friend replies, “But you hit me with exactly the same force as I hit you!” Is your friend correct? Explain your answer.
Exercise 6.5 Newton’s laws of motion Newton’s laws summarise much of the material covered in the first six chapters of this book. This exercise is designed to check your understanding of these important laws. 1 This question is about Newton’s first law of motion. a An object is stationary. No force acts on it. What does Newton’s first law say about its motion? b An object is moving. No force acts on it. What does Newton’s first law say about its motion? c An object is moving. It is acted on by four forces whose resultant force is zero. What does Newton’s first law say about its motion? d An object is moving with constant velocity. What does Newton’s first law say about the forces acting on it? e An object is moving at a steady speed along a curved path. What does Newton’s first law say about the forces acting on it? 2 This question is about Newton’s second law of motion. a An object is moving in a straight line with constant acceleration. i
What can you say about the rate of change of the object’s momentum?
ii What does Newton’s second law say about the forces acting on the object? b A skydiver is falling towards the Earth. Her velocity is increasing but her acceleration is decreasing. i
What does Newton’s second law say about the forces acting on the skydiver? Think about the skydiver’s momentum and the rate at which it is changing.
ii Two forces act on the skydiver as she falls. Referring to these forces, explain how her velocity can be increasing while her acceleration is decreasing. 3 We can write Newton’s second law using SI units: resultant force = rate of change of momentum a Explain why we have to say ‘in SI units’. b Use the equation to express the newton (N) in terms of SI base units.
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