Hobson Physics: Concepts & Connections 4e

Chapter 3 review questions Aristotelian Physics 1. Describe the four kinds of motion that Aristotle considered to be natural on Earth. 2. Give two examples that Aristotle considered to be violent motions. 3. According to ancient Greek thought, in what fundamental way does Earth differ from the heavens? 4. Give an example of a motion that contradicts Aristotelian physics. 5. According to Aristotelian physics, why does a stone fall when it is released above the ground? According to Newtonian physics? 6. Describe at least one principle of Aristotelian physics that seems intuitively plausible but that Newtonian physics rejects. The Law of Inertia 7. What does it mean to say that an object has inertia? 8. What does the law of inertia say about the velocity of a body that is subject to no external influences? What does this law say about such a body’s acceleration? 9. Which of the following describes how high you must go before you will first reach “outer space”: anywhere above the ground, about 100 m, about 1 km, about 100 km, beyond the moon, beyond the solar system? 10. Is there air in outer space? 11. Give at least one example that demonstrates, at least approximately, the law of inertia—in others words, an example of unassisted motion at constant, or nearly constant, velocity. Speed and Velocity 12. Describe how you could use a clock and a meter stick to measure a moving object’s speed. 13. When we say “5 centimeters per second,” what does the “per” mean? 14. What is the difference between speed and average speed? In what circumstances are they the same? 15. Can you give an example in which the speed is unchanging but the velocity changes? If so, give one. 16. Can you give an example in which the velocity is unchanging but the speed changes? If so, give one. Acceleration 17. A car speeds up along a straight line. Describe how you could use clocks and meter sticks to measure its acceleration. 18. How is acceleration related to velocity? 19. If an object’s position is changing, can we be certain that it has a nonzero velocity? Can we be certain that it has a nonzero acceleration? 20. If an object is moving in a circle at an unchanging speed, is it accelerated? 21. If an object is slowing down, is it accelerated? Falling 22. An object is released above the ground and falls freely. At which of the following places during the fall is its velocity greatest: the top, the midpoint, or a point near the bottom? At which position is its acceleration greatest? 23. What is the meaning of the phrase “acceleration due to gravity”? What is its approximate value on Earth? 24. What does “speed is proportional to time” mean? 25. In twice the time, does a freely falling object fall (from rest) twice as far? Does it gain twice as much speed? conceptual exercises Aristotelian Physics 1. You roll a ball. It soon rolls to a stop. How would Aristotle interpret this? How would Galileo interpret it? The Law of Inertia 2. Most meteoroids—pebble-sized to boulder-sized rocks in outer space—have been moving for billions of years. What, if anything, keeps them moving? 3. If you ride on a smooth, fast train at an unchanging speed and throw a baseball upward inside the train, will the baseball then get left behind and come down toward the rear of the car? Explain. 4. If a ball is moving at 20 m/s and no forces ever act on it, what will its speed be after 5 s? After 5 y? 5. Do you suppose that the photo of Earth shown in Figure 1.7 was taken from a low-orbit satellite (see Figure 3.7)? 6. In order to experimentally verify the law of inertia, would you need to be able to measure time? Weight? Distance? 7. When a moving bus comes rapidly to a stop, why do the riders who are standing lurch toward the front of the bus? Speed and Velocity 8. Can you drive your car around the block at a constant velocity? 9. Mary passes Mike from behind while bicycling. As she passes him, do the two have the same velocity? The same speed? 10. Mary is bicycling straight north at 15 km/hr, and Mike is bicycling straight south at 15 km/hr. As they pass each other, do they have the same velocity? The same speed? 11. Figure 3.13 represents a multiple-flash photo of two balls moving to the right. The figure shows both balls at several numbered times. The flash times are equally spaced. Which ball has the greater acceleration? The greater speed? The greater velocity? Does either ball pass the other, and if so, when? 12. The automobile beltway around many cities is approximately circular. Suppose that you start driving at point A on the east side of the beltway and drive counterclockwise at an unchanging 90 km/hr (Figure 3.14). As you pass point B on the north side, what is your speed? Your velocity? 13. Referring to the preceding exercise, as you pass point C on the west side, what is your speed? Your velocity? 14. In Figure 3.13, suppose the large divisions on the measuring rod are centimeters and that the time intervals each have a duration of 0.20 s. Find the speed of each ball. 15. Find the average speed of a jogger who jogs 3 km in 15 min. Give your answer in km/hr. Acceleration 16. A French TGV train cruises on straight tracks at a steady 290 km/hr (180 mi/hr). What is its acceleration? 17. Is the “motion sickness” that some people get in a car actually due to motion per se or to something else? Describe one form of motion that would not make people sick. 18. When you drive a car, might you depress the accelerator pedal without actually accelerating? Could you accelerate without having your foot on the accelerator? Explain. 19. One car goes from 0 to 30 km/hr. Later another car goes from 0 to 60 km/hr. Can you say which car had the greater acceleration? Explain. 20. Figure 3.15 represents a multiple-flash photo of two balls. Describe each ball’s motion. Does either ball pass the other? When? Do they ever have the same speed? When? 21. In each of the following cases, is the motion accelerated or not accelerated? (a) A rock falling freely for 2 m. (b) A meteoroid (a rock in outer space) that is so far from all planets and stars that gravity is negligible. (c) An artificial satellite orbiting Earth at a steady 30,000 km/hr. (d) The moon. (e) An ice-skater coasting on smooth ice, neglecting friction and air resistance. 22. Can a slow-moving object have a large acceleration? Can a fast-moving object have a small acceleration? 23. Which devices in a car are designed to cause acceleration? 24. For an unassisted (unforced, or isolated) moving object, which of the following quantities change, and how do they change: distance (from the starting point), speed, velocity, acceleration? 25. An automobile moves along a straight highway at an unchanging 80 km/hr. During the motion, which of the following quantities change, and how do they change: distance (from the starting point), speed, velocity, acceleration? 26. A ball rolls down a straight ramp. During the motion, which of the following quantities change, and how do they change: distance (from the starting point), speed, velocity? 27. A bicyclist increases her speed along a straight road from 3 m/s to 4.5 m/s, in 5 s. Find her acceleration. 28. A car accelerates from 0 to 100 km/hr in 10 s. Find its acceleration. Drag racers can get to 400 km/hr from rest in 5 s. How big is this acceleration? 29. Find the acceleration of a car as it speeds up from 70 to 82 km/hr in 4 s. From 70 to 82 km/hr in 16 s. From 70 to 94 km/hr in 8 s. From 70 to 76 km/hr in 8 s. Falling 30. Multiple choice: Two metal balls are dropped from a third-story window at the same time. They are the same size, but one weighs twice as much as the other. The time to reach the ground will be (a) about twice as long for the heavy ball; (b) about twice as long for the light ball; (c) about the same for both; (d) considerably longer for the heavy ball, but not necessarily twice as long; (e) considerably longer for the light ball, but not necessarily twice as long. 31. Figure 3.12 shows that a falling object has a speed of 10 m/s at So it seems that it should move 10 m in 1 s. Yet the data say that the object moves only 5 m in 1 s. What is wrong here? 32. Figure 3.16 represents a multiple-flash photo of a falling ball. Neglect air resistance. At which point, A or B, is the ball’s acceleration larger? At which point is its velocity larger? 33. By how much does a freely falling object’s speed increase during its third second of fall (from to after release)? During its fourth second of fall? 34. For an object that is freely falling to Earth, which of the following quantities increase during the fall: distance (from the starting point), speed, velocity, acceleration? 35. As an object falls freely, what will be its speed at the end of the first second? Second second? Third second? 36. As an object falls freely, what will be its acceleration at the end of the first second? Second second? Third second? 37. An astronaut on another planet, one that has no atmosphere, drops a rock off a cliff. How much faster is the rock moving at the end of 3 s as compared with 1 s? How much farther (measured from the release point) does the rock fall in 3 s as compared with the distance it falls in 1 s? 38. Neglecting air resistance, would the answers to the preceding exercise be different if the rock is dropped on Earth? Neglecting air resistance, would the distance fallen in 3 s on Earth be likely to be the same as the distance fallen in 3 s on the other planet? Figure 3.13 Figure 3.14 Figure 3.15 Figure 3.16 problems Velocity 1. Worldwide sea levels are currently rising at about 2 mm per year, probably because of global warming. At this rate, how long will it be before sea levels have risen by 0.5 m? 2. It takes light about 8 minutes to travel here from the sun. Given that the speed of light is 300,000 km/s, how far is it to the sun? 3. It is to the moon. How long does it take a radar beam, traveling at the speed of light (300,000 km/s), to get from Earth to the moon and back? 4. You wish to travel from downtown New York City (NYC) to downtown Washington DC (DC), a distance of 330 km. You consider two options: train and plane. The high-speed train takes 1.5 hr, plus 30 min in stations. The airplane flies the 330 km (airport to airport) in just 30 min, but the drive to the NYC airport takes 30 min, you must arrive 2 hr before departure time, the plane waits 15 min for takeoff, and it takes 45 min to get your luggage and drive into DC. Find the train’s track speed, the plane’s flying speed, the total travel time for each option, and the overall average speed for each option. 5. You drive from New York City to Washington DC by car. You drive in traffic for the first hour at an average 50 km/hr. You cover the next 250 km in 3.0 hr, and then drive the remaining 30 km into Washington in 30 min. Find your total time and average speed. Acceleration (You will need footnote 5 to solve Problems 6 through 17.) 6. A car starts from rest and maintains an acceleration of 4.5 (km/hr)/s for 5 s. How fast is it going at the end of the 5 s? 7. A car is moving at 30 km/hr. The driver then presses harder on the accelerator, causing an acceleration of 2.25 (km/hr)/s, which she maintains for 4 s. How fast is the car going at the end of the 4 s? Falling 8. You drop a rock off a cliff and note that it hits the ground below in 6 s. How high is the cliff, assuming that the air is so thin that air resistance can be ignored during the entire fall? Unless the air is extremely thin, air resistance will not be negligible. What does this tell you about your calculation of the cliff’s height: Is the cliff actually higher, or is it lower, than your calculated answer? 9. In the preceding problem, how fast is the rock moving when it hits the ground, still assuming negligible air resistance? 10. You drop a rock down a well and hear a splash 3 s later. As Charlie Brown would say, the well is “3 seconds deep.” But how many meters deep is it, assuming that air resistance is negligible and that the time for sound to travel back up the well is also negligible? 11. In the preceding problem, how fast is the rock moving when it hits the water? 12. You drop an apple out of a third-story window. When does it pass the second-story window 4 m below? 13. When does the apple in the preceding question hit the ground 8 m below? 14. A car speeds up from rest, along a straight highway. Its acceleration is unchanging. How much farther (measured from the starting point) does it get in 10 s, as compared with 1 s? 15. In the preceding problem, how much faster is it moving in 10 s, as compared with 1 s? 16. On the planet Mars, a free-falling object released from rest falls 4 m in 1 s and is moving at 8 m/s at that time. How fast would such an object be moving after 2 s? 3 s? 17. In the preceding problem, how far would such an object fall in 2 s? 3 s?