Practice TEST 2 1. Imagine taking an elevator ride from the1 st floor to the 10 th floor of a building. While moving between the 1 st and 2 nd floors the elevator speeds up, but then moves at a constant speed between the 2 nd and 9 th floors. Which one of the following statements about the elevator best describes the forces acting on it, as it moves upward at a constant speed? (Assume any frictional forces can be neglected.) Circle your choice. a) The upward pull of the cable is stronger than the downward gravitational pull of the Earth. b) The upward pull of the cable is equal in strength to the downward gravitational pull of the Earth. c) The upward pull of the cable is weaker than the downward gravitational pull of the Earth. Explain your reasoning When an object moves at a constant speed the forces acting on it must be balanced. In this case the only two forces involved are the downward gravitational pull of the Earth and the upward pull of the cable. For the forces to be balanced these two must be equal in strength. (If one of the forces was stronger than the other, the forces would be unbalanced and the speed of the elevator would be changing.) 2. As the elevator in the previous question passes the 9 th floor it begins to slow down (still moving upward), before stopping at the 10 th floor. As the elevator is slowing down, which one of the following statements best describes the forces acting on it? (Again, assume any frictional forces can be neglected.) Circle your choice. d) The upward pull of the cable is stronger than the downward gravitational pull of the Earth. e) The upward pull of the cable is equal in strength to the downward gravitational pull of the Earth f) The upward pull of the cable is weaker than the downward gravitational pull of the Earth. Explain your reasoning In order for an object to slow down, the force acting in the opposite direction to the motion must be stronger than any force acting in the same direction as the motion. Since the elevator is slowing down as it moves upward, this means the downward force must be stronger than the
upward force. The only way for this to happen is if the upward pull of the cable is weaker than the downward gravitational pull of the Earth. 3. Consider two different carts that are designed to move along a surface with essentially no friction. Cart A is has more mass than cart B. A student performs two experiments with these carts. In the first experiment she mounts identical fans (with same number of batteries) on each of the two carts and puts the carts on separate tracks. She turns both fans on and lets the carts start moving at the same time. To the right is a force diagram (with motion arrow) for cart B one second after the carts begin moving: Consider cart A (the more massive one). Suppose you wanted to draw a force diagram for cart A at the same time (one second after starting), including both a force arrow and a speed arrow: g) Which of the following arrows could best represent the length of the force arrow on cart A. Circle your choice and briefly justify why you made that choice. Since the two fans are identical (with the same number of batteries), they will push with equal force strengths. Therefore the force arrow on both carts should be the same length. h) Which of the following arrows could best represent the length of the speed arrow for cart A. Circle your choice and briefly justify why you made that choice.
Cart A has more mass than Cart B, but it has the same strength force acting on it. Therefore Cart A will speed up at a slower rate than Cart B, and so at 1 second after starting it will be moving more slowly than Cart B. This slower speed would be represented by a shorter speed arrow. 4. An archer uses a bow to shoot an arrow straight up into the air on a still, calm day. The arrow rises to a certain height and then falls straight back down, sticking in the ground. The speed time graph for the arrow shown below is for the period starting just after it leaves the bow to just before it sticks in the ground. i) At what time does the arrow reach its highest point? Briefly justify how you know. The arrow reaches its highest point at 5 seconds. We can tell this from the graph because it is at this time that it stops slowing down as it rises, stops for an instant at its highest point, and then begins to speed up again as it falls. j) Indicate on the graph the entire region during which the gravitational potential energy of the arrow+earth system was increasing. Briefly justify how you know. The gravitational potential energy in the system was increasing while the arrow was slowing down on its way upward. Since the arrow was slowing down, the kinetic energy in the system was decreasing. Therefore, we know from our ideas of energy conservation that the potential energy must be increasing to compensate for this. k) During the entire time that the arrow was in flight (up and down) what was happening to the total energy (kinetic + gravitational potential) of the arrow+earth system. Was it increasing, decreasing, or staying constant? How do you know?
Since energy cannot be created or destroyed, and assuming there is no energy input or output to the arrow+earth system, then the total energy in the system stayed constant. Any changes in kinetic energy were exactly compensated for by equal and opposite changes in gravitational potential energy. In the remainder of this problem you will explain why the arrow slows down after it leaves the bow and until it reaches its highest point. You will first explain it in terms of energy ideas, and then will explain it again in terms of force ideas. In both cases, ignore the effects of air resistance. Explanation in terms of energy ideas: Why does the arrow slow down as it rises? Draw an I/O energy diagram for the arrow+earth system as the arrow is rising. Earth And Arrow Decrease in kinetic energy Increase in gravitational potential energy Write a few sentences to explain (in energy terms) why the arrow slows down as it rises. After the arrow leaves the bow, the only interaction it is involved in as it moves upward is the gravitational interaction with arrow and the Earth. As the arrow rises it gets further from the Earth and so the gravitational potential energy in the arrow+earth system increases. Because there is no energy input to, or output from, this system, the Law of Conservation of Energy tells us that this increase in gravitational potential energy must be compensated for by a decrease in the kinetic energy of the objects in the system. Thus, since the kinetic energy of the arrow decreases as it rises, this means it slows down.
Explanation in terms of force ideas: Why does the arrow slow down as it rises? Complete the force diagram for this interaction: (Draw both force and motion arrows. Label the force arrow.) Direction of motion Gravitational force of the Earth Write a few sentences to explain (in force terms) why the arrow slows down as it rises. 5. The gravitational force of the Earth acts downward on the arrow. As the arrow rises, this force is acting in the opposite direction to its motion. When a force acts in the opposite direction to the motion of an object, its speed decreases. Therefore, the speed of the arrow decreases as it rises. Fill the blanks in the system
MULTIPLE CHOICE QUESTIONS 1) How many different elements are in a water molecule? A) one B) two C) three D) four E) none 2) An atomic mass unit (amu) is 1/12 the mass of A) an electron. B) a proton. C) a hydrogen atom. D) a carbon atom. E) a uranium atom. Answer: D 3) Which of the following statements is true?
A) An atom is the smallest particle known to exist. B) There are only about 100 different kinds of atoms that combine to form all substances. C) There are thousands of different kinds of atoms that account for a wide variety of substances. D) A large atom can be photographed with the aid of an ordinary microscope. E) None of these statements are true. 4) Which of the following is not a mixture? A) granite B) cake C) air D) beach sand E) None. All of the above choices are mixtures. Answer: E 5) Solid matter is mostly empty space. The reason solids don't fall through one another is because A) atoms are constantly vibrating, even at absolute zero. B) of nuclear forces. C) of gravitational forces. D) of electrical forces. Answer: D 6) Compared to the energy it takes to separate oxygen and hydrogen from water, the amount of energy given off when they recombine is A) slightly more. B) slightly less. C) much more. D) much less. E) the same. Answer: E 7) Compared to the atoms that make up the body of an elderly person, the atoms that make up the body of a newborn baby are A) newer. B) actually older. C) the same age. 8) The chemical properties of matter come mostly from its A) protons. B) electrons. C) neutrons.
9) If a gram of antimatter meets a kilogram of matter, the amount of mass to survive is A) 1 gram. B) 999 grams. C) 1 kilogram. D) 1.1 kilogram. 10) If two protons are added to an oxygen nucleus, the result is A) heavy oxygen. B) fluorine. C) neon. D) sodium. E) nitrogen. 11) Which of these atoms has the greatest amount of electrical charge in its nucleus? A) helium B) carbon C) iron D) gold E) uranium Answer: E 12) Which of these atoms has the greatest number of electrons? A) helium B) carbon C) iron D) gold E) uranium Answer: E 13) Two objects have the same size and shape, but one is much heavier than the other. When they are dropped simultaneously from a tower, they reach the ground at the same time, but the heavier one has a greater A) speed. B) acceleration. C) momentum. D) all of these 14) To catch a ball, a baseball player extends the hand forward before impact with the ball and then lets it ride backward in the direction of the ball's motion. Doing this reduces the force of impact on the player's hand principally because the A) force of impact is reduced. B) relative velocity is less.
C) time of impact is increased. D) time of impact is decreased. 15) When you are in the way of a fast-moving object and can't get out of its way, you will suffer a smaller force of impact if you decrease its momentum over a A) long time. B) short time. C) same way either way 16) A bullet is fired from a gun. The speed of the bullet will be about the same as the speed of the recoiling gun A) because momentum is conserved. B) because velocity is conserved. C) because both velocity and momentum are conserved. D) if the mass of the bullet equals the mass of the gun. Answer: D 17) The force on an apple hitting the ground depends upon A) the speed of the apple just before it hits. B) the time of impact with the ground. C) whether or not the apple bounces. D) all of these Answer: D 18) If a monkey floating in outer space throws his hat away, the hat and the monkey will both A) move away from each other, but at different speeds. B) move away from each other at the same speed. C) move a short distance and then slow down. D) move a short distance and then go faster. E) come to a stop after a few minutes. 19) A sandbag is motionless in outer space. A second sandbag with 3 times the mass moving at 12 m/s collides with it. They stick together and move at a speed of A) 3 m/s. B) 4 m/s. C) 6 m/s. D) 8 m/s. Answer: E 20) An object may have potential energy because of its
A) speed. B) acceleration. C) momentum. D) location. Answer: D 21) A bow is drawn so that it has 40 J of potential energy. When fired, the arrow will ideally have a kinetic energy that is A) less than 40 J. B) more than 40 J. C) 40 J. D) impossible to predict without additional information 22) It takes 40 J to push a large box 4 m across a floor. Assuming the push is in the same direction as the move, what is the magnitude of the force on the box? A) 4 N B) 10 N C) 40 N D) 160 N 23) Using 1000 J of work, a toy elevator is raised from the ground floor to the second floor in 20 seconds. The power needed to do this job was A) 20 W. B) 50 W. C) 100 W. D) 1000 W. E) 20,000 W. 24) An object at rest may also have A) speed. B) velocity. C) momentum. D) kinetic energy. E) potential energy. Answer: E 25) Two pool balls, each moving at 2 m/s, roll toward each other and collide. Suppose after bouncing apart, each moves at 4 m/s. This collision violates conservation of A) momentum. B) energy. C) both momentum and energy.
D) none of the above choices 26) If several balls are thrown straight up with varying initial velocities, the quantity that will have the same value for each trial is the ball's A) initial momentum. B) maximum height. C) time of travel. D) acceleration. E) None of the above choices are correct. Answer: D 27) The rotational inertia of your leg is greater when your leg is A) straight. B) bent. C) same either way 28) On a balanced seesaw, a boy three times as heavy as his partner sits A) 1/3 the distance from the fulcrum. B) less than 1/3 the distance from the fulcrum. C) more than 1/3 the distance from the fulcrum. 29) Two people are balanced on a seesaw. If one person leans toward the center of the seesaw, that person's end of the seesaw will A) rise. B) fall. C) stay at the same level. D) rise and then fall. E) fall and then rise. 30) A ball rolls down a hill mainly because of A) an unbalanced torque. B) a balanced torque. C) its rotational inertia. D) its angular acceleration. E) its angular momentum. 31) Suppose you are at the center of a large freely-rotating horizontal turntable in a carnival funhouse. As you crawl toward the edge, the angular momentum of you and the turntable A) decreases. B) increases. C) remains the same, but the RPMs decrease.
D) decreases in direct proportion to your decrease in RPMs. 32) Stand a meterstick on its end and let go and it rotates to the floor. If you attach a heavy weight to its upper end and repeat, falling time will be A) more. B) less. C) the same. 33) A boy plays solitary seesaw by placing a long plank over a small rock and sitting at one end of the plank. When the seesaw is balanced, the boy's mass is most likely A) greater than the mass of the seesaw. B) less than the mass of the seesaw. C) equal or very nearly equal to the mass of the seesaw. D) Not enough information is given. 34) An asteroid exerts a 360-N gravitational force on a nearby spacecraft. If the spacecraft moves to a position three times as far from the center of the asteroid, the force will be A) zero. B) 40 N. C) 120 N. D) 360 N. E) 1080 N. 35) Inside a freely falling elevator, there would be no A) gravitational force on you. B) apparent weight for you. C) both of these D) none of these 36) No force due to Earth's gravity is acting on the astronauts inside the orbiting space shuttle. This statement is A) always true while in orbit. B) sometimes true while in orbit. C) always false. 37) From Earth, one satellite appears to overtake another. The faster satellite is A) higher. B) lower. C) smaller.
D) can't say 38) Acceleration is greater for a satellite when it is at the A) apogee. B) perigee. C) zenith. D) same acceleration at all of the above places. 39) Angular momentum is greater for a satellite when it is at the A) apogee. B) perigee. C) same at apogee and perigee 40) The period of a satellite, the time it takes for a complete revolution, depends on the satellite's A) mass. B) weight. C) radial distance. D) all of these 41) Minimal orbital speed about the Earth is about 8 km/s. Minimal orbital speed about the moon would be A) less than 8 km/s. B) more than 8 km/s. C) about 8 km/s. 42) The speeds of the planets about the sun depend on A) their distances from the sun. B) the masses of the planets. C) their periods of rotation. D) None of the above are correct. 43) According to Newton, doubling the distance between two interacting objects A) divides by 2 the gravitational force between them. B) multiplies by 2 the gravitational force between them. C) divides by 4 the gravitational force between them. D) multiplies by 4 the gravitational force between them. 44) The concept of force is not fundamental to
A) Newton's theory of gravitation. B) Einstein's theory of gravitation. C) both of these D) neither of these 45) An asteroid exerts a 360-N gravitational force on a nearby spacecraft. If the spacecraft moves to a position three times as far from the center of the asteroid, the force will be A) zero. B) 40 N. C) 120 N. D) 360 N. E) 1080 N. 46) If the radius of the Earth somehow decreased with no change in mass, your weight would A) increase. B) not change. C) decrease.