Hobson Physics: Concepts & Connections 4e

Chapter 4 review questions Force 1. How can we tell whether a body is exerting a force on another body? 2. Can an object have force? Can an object exert a force? Can an object be a force? Can an object feel a force? 3. List at least six specific examples of forces. Try to list examples that are significantly different. 4. What is a resistive force? Give two examples. 5. Give two examples of forces that act at a distance. Newton’s Law of Motion 6. What does Newton say that forces cause? What does Aristotle say? 7. What do we mean when we say that one object has “more inertia” than another object does? 8. When you move an object from Earth to the moon, does its inertia change? Does its weight change? Does its mass change? Does its amount of matter change? Does its acceleration differ while falling freely? Does it respond differently to a force of 1 N? 9. Forces of 8 N and 3 N act on an object. How strong is the net force if the two forces have opposite directions? The same directions? 10. Is an object’s acceleration always in the same direction as its speed (its direction of motion)? If not, give an example in which it is not. Is an object’s acceleration always in the same direction as the net force on the object? If not, give an example in which it is not. 11. As you increase the net force on an object, what happens to its acceleration? What if you double the net force? As you increase the mass of an object (for example, by gluing additional matter to it), what happens to its acceleration? What if you double the mass? Weight 12. What is weight? Is it the same as mass? If not, what is the difference? 13. Describe a simple way to determine, in a lab, whether two objects have equal masses. Would this method work in distant space? What would work in distant space? 14. Find the gravitational force on a 1 N apple. Would it still weigh 1 N if we took it to the moon? 15. Draw a force diagram showing the forces on an apple at rest on a table. Find the net force on the apple. 16. Draw a force diagram showing the forces on a rocket during liftoff. Which force is largest? What is the direction of the net force? 17. Where is it easiest to lift your automobile: on Earth or on the moon? Where is the automobile’s mass larger? Law of Force Pairs 18. Describe several experiments demonstrating that forces come in pairs. 19. Do you exert a gravitational force on Earth? How do you know? What direction is this force? 20. Describe the other member of the force pair for each of the following forces: the normal force on a book, the weight of an apple, the force by a bat against a baseball, the force by a baseball hitting a catcher’s mitt. The Automobile 21. Describe four examples of forces acting on “self-propelled” objects. 22. Draw a force diagram showing the forces on a car driving along a straight, level road. How would this force diagram be altered if the car were coasting? What if the car were braking? 23. What is the main difference between propeller-driven airplanes and jet airplanes? 24. How does the forward force on a car compare with the resistive forces when the car maintains a constant speed? When the car is speeding up? Slowing down? 25. When a car moves at constant speed along a straight road, is the forward force (the force that moves the car forward) zero? Is the net force zero? Is the acceleration zero? Is the speed zero? 26. What is the main difference between the force that propels a rocket and the forward force that propels airplanes and automobiles? conceptual exercises Force 1. Is any force exerted on you when you speed up along a straight line? When you slow down along a straight line? How do you know? 2. Is any force exerted on you while you move in a circle at unchanging speed? How do you know? 3. A smooth ball rolls on a smooth table. Initially, no horizontal forces are exerted on the ball. Then you bring a magnet near the rolling ball, but you are not sure whether the magnet actually exerts a magnetic force on the ball. How can you tell whether the magnet is exerting a horizontal force on the ball? 4. Does a high-speed bullet contain force? Does a stick of dynamite contain force? Newton’s Law of Motion 5. You place your book on a table and hit it horizontally with a hammer, strongly but briefly. Do not neglect friction. Describe the motion of the book, beginning from just before you hit it with the hammer. Describe the direction and strength of the net force on the book during the entire motion. 6. If you exert a force on an object and then exert three times as strong a force on the same object, what (if anything) can you say about the object’s acceleration during the exertion of each force? 7. A ball weighing 8 N is thrown straight upward. Disregarding air resistance, find the direction and strength of the net force on the ball as it moves upward. What is the direction of the ball’s acceleration? Are the net force and the acceleration in the same direction in this case? Can they ever be in different directions? 8. An object moves with unchanging speed in a straight line. Does it then have no forces acting on it? Explain. Does it have no net force acting on it? 9. An object is at rest. Does it then have no forces acting on it? Explain. Does it have no net force acting on it? 10. When you stand on the floor, does the floor exert a force on your feet? In which direction? Why, then, don’t you accelerate in that direction? 11. You push on a solid concrete wall. Is your push the only horizontal force on the wall? How do you know? What can you say about the net force on the wall? 12. A car starts up from rest, moving along a straight highway with an acceleration of A second car comes racing past at a steady 120 km/hr. Which car has the larger net force acting on it? 13. An 80 tonne truck and a 1 tonne car maintain a steady speed of 80 km/hr on a straight highway. Which vehicle has a larger net force exerted on it? Which vehicle has the larger normal force exerted on it? 14. An 80 tonne truck and a 1 tonne car maintain a steady speed of 80 km/hr on a straight highway. Which vehicle has the larger drive force exerted on it? The larger air resistance force? The larger net force? 15. A 3 kg rock rests on the ice. You kick it, briefly exerting a 60 N force. Find the rock’s acceleration, assuming that there is no friction. Still assuming no friction, what will be the rock’s acceleration after your foot is no longer in contact with the rock? Will the rock have a (nonzero) speed at this time? 16. In the preceding question, assume now that a frictional force of 6 N acts on the rock whenever it is moving across the ice. Find the net force on the rock and the rock’s acceleration. What can you say about the net force on the rock after your foot is no longer in contact with it? 17. Ned the skydiver weighs 600 N and has a mass of 60 kg. How large must be the force of air resistance acting on Ned in order for Ned to maintain an unchanging speed while falling through the air? 18. In the preceding question, what would be Ned’s acceleration if there were no air? 19. A car weighing 8000 N moves along a straight, level road at a steady 80 km/hr. The total resistive force on the car is 500 N. Find the net force on the car and the acceleration of the car 20. In the preceding question, find the drive force on the car. Weight 21. Roughly, what is your weight in newtons? 22. Which has the greater mass, a tonne of feathers or a tonne of iron? Which has the greater weight? Which has the larger volume? 23. Would you rather have a hunk of gold whose weight is 1 N on the moon or one whose weight is 1 N on Earth—or wouldn’t it make any difference? 24. Would you rather have a hunk of gold whose mass is 1 kg on the moon or one whose mass is 1 kg on Earth—or wouldn’t it make any difference? 25. A standard kilogram in your physics lab weighs (approximately) 10 N, or 2.2 pounds. What are its mass and weight in distant space? 26. Find the strength and direction of the net force on an apple weighing 2 N, neglecting air resistance, in each of the following cases: The apple is held at rest in your hand. The apple is falling to the ground. The apple is moving upward, just after you threw it upward. 27. An apple is accelerated upward by your hand. Which is larger, the apple’s weight or the upward force by your hand? What if you accelerate the apple downward while it is in the palm of your hand? What if you lift the apple at an unchanging velocity? What if you lower the apple at an unchanging velocity? 28. Would it be easier (in other words, would it require less thrust and less rocket fuel) to lift a rocket off the moon’s surface than off Earth’s surface? Why? 29. An astronaut on the moon picks up a large rock. Would it be easier, or harder, or neither for him to pick up the same rock on Earth? 30. An astronaut on the moon kicks (horizontally) a large rock. What if she kicked the same rock on Earth? Neglecting frictional effects, would it hurt her foot more, or less, or just as much? 31. Neglecting friction and air resistance, would it be easier to set this book into horizontal motion at 5 m/s on Earth, or on the moon, or in distant space? Law of Force Pairs 32. “Planet Earth is pulled upward toward a falling boulder with just as much force as the boulder is pulled downward toward Earth.” True or false? Why? 33. “Planet Earth is pulled toward a falling boulder with just as much acceleration as the boulder has as it moves toward Earth.” True or false? Why? 34. A large truck breaks down on the highway and receives a push back into town by a small car. While moving at unchanging speed, does the car exert any force on the truck? Does the truck exert any force on the car? If so, is this force weaker or stronger than the force that the car exerts on the truck? 35. A car collides head-on with a large truck. Which vehicle exerts the stronger force? Which has the larger force exerted on it? Which experiences the larger acceleration? 36. When a rifle fires, it accelerates a bullet along the barrel. Explain why the rifle must recoil. 37. A 2 N apple hangs by a string from the ceiling. Describe the two forces on the apple. How strong is each of these forces? Do these forces form a single force pair? If not, then for each force, describe the other member of that force’s force pair. 38. A horizontally moving bullet slows down. Is anything exerting a force on it? How do you know? Is it exerting a force on anything? How do you know? 39. I push you away from me. Do you also push (exert a force on) me? Which force is stronger—or does it depend on which of us is heavier? 40. A pitcher exerts a force on a baseball while throwing it. Describe the other member of the force pair. 41. A rope pulls forward on a water skier. Describe the other member of the force pair. 42. As we know, “weight” is a force, and force is an interaction. In the case of your own weight, name the two objects that are involved in this interaction. 43. Describe the two forces that act on a book that rests in the palm of your hand. Are these two forces equal but opposite to each other? Are these two forces part of one force pair? 44. Continuing the preceding question, suppose you accelerate the book into upward motion. How many forces act on it? Are these two forces equal but opposite to each other? 45. A freely falling apple has a weight of 1 N. Earth’s mass is How strong is Earth’s force on the apple? 46. In the preceding question, how strong a force does the apple exert on Earth? 47. Still continuing the preceding question, how big is the apple’s acceleration? Find the acceleration that the apple would cause Earth to have if the apple was the only object exerting a force on Earth. The Automobile 48. Since the law of inertia states that no force is needed to keep an object moving in a straight line at an unchanging speed, why is a force needed to keep a car moving? 49. While driving your car on a straight, level road, you slam on your brakes. Draw a force diagram of the car during braking. What is the direction of the net force? Draw a force diagram for a car that is coasting without braking. In which of the two cases is the net force stronger? 50. Why is it easier to pedal a bicycle with hard high-pressure tires as compared with soft balloon tires? 51. When you hold your foot on a car’s accelerator pedal, is the car necessarily accelerating? Could it be accelerating? Could it have a forward acceleration? Could it have a backward acceleration? 52. There are three acceleration devices on any car. What are they, and what kinds of acceleration does each one give to the car? 53. If a jet airplane were above Earth’s atmosphere, could it then accelerate? What about a rocket-driven plane? 54. Magnetic forces can levitate railroad trains a short distance above the tracks, making friction practically negligible. Suppose such a “maglev” train runs inside an evacuated (emptied of most air) tunnel from New York City to Chicago. If friction and air resistance are negligible, during what parts of the trip would an external horizontal force act on the train? Discuss the direction of this force during each part of the trip. problems Newton’s Law of Motion 1. You push on a 2 tonne (2000 kg) vehicle on level pavement with a force of 250 N. Find the vehicle’s acceleration. 2. How large is the acceleration of a 60 kg runner if the friction between her shoes and the pavement is 500 N? 3. In order for a 60 kg runner to accelerate at what must be the frictional force between her shoes and the pavement? 4. A 747 jumbo jet of mass 30,000 kg accelerates down the runway at What must be the thrust of each of its four engines? 5. What would a skydiver’s acceleration be if air resistance was half as large as the skydiver’s weight? What if air resistance were as large as the skydiver’s weight? 6. How much force must a pitcher exert on a 0.5 kg baseball in order to accelerate it at 7. Find the force acting on a 0.01 kg bullet as it is accelerated at (100,000 times larger than the acceleration due to gravity!) down a rifle barrel. 8. A 2 kg flower pot weighing 20 N falls from a window ledge. How large must air resistance be in order that the pot fall with an acceleration of 9. An 80 kg firefighter whose weight is 800 N slides down a vertical pole with an acceleration of What is the frictional force on the firefighter? 10. A black box and a white box accelerate at the same rate across the floor despite the fact that the net force on the black box is four times larger than the net force on the white box. Which box has the larger mass, and how much larger? 11. A 70 kg runner speeds up from 6 m/s to 7 m/s in 2 s. Find the runner’s acceleration and the frictional force by the ground on the runner during this time. 12. A 1 tonne (1000 kg) automobile experiences 100 N of air resistance and 200 N of rolling resistance. How large a forward force must the road exert on the drive wheels in order for the automobile to accelerate at The Law of Force Pairs 13. Wearing frictionless roller skates, you push horizontally against a wall with a force of 50 N. How hard does the wall push on you? 14. In Problem 13, if your mass is 40 kg, then what is your acceleration? 15. Your friend (mass 80 kg) and you (mass 40 kg) are both wearing frictionless roller skates. You are at rest, behind your friend. You push on your friend’s back with a force of 60 N. How hard does your friend’s back push on you? 16. In the preceding question, what is your acceleration? What is your friend’s acceleration? 17. A small car having a mass of 1000 kg runs into an initially stationary 60,000 kg 18-wheeled truck from behind, exerting a force of 30,000 N on the truck. How big, and in what direction, is the force that the truck exerts on the car? 18. In the preceding question, find the car’s acceleration. Is this a “speeding up” or a “slowing down” type of acceleration? Find the truck’s acceleration. Is this of the “speeding up” or “slowing down” variety? 19. You press downward with a 100 N force on a brick weighing 40 N that rests on a table. With what force, and in what direction, does the brick press against your hand? Draw a force diagram similar to Figure 4.16, with an arrow representing each force acting on the brick. 20. In the preceding question, how big is the net force on the brick? Find the force (how big and in what direction) that the table exerts on the brick. How hard is the brick pressing down against the table?