Physic Force Homework

1.Is acceleration proportional to net force, or does acceleration equal net force? Yes, acceleration is proportional to net force. 2. When you push horizontally on a crate on a level floor that doesn’t slide, how great is the force of friction on the crate? The frictions is the same strength as the force you pushed the crate. 3. As you increase your push, will friction on the crate increase also? Yes, because as the crate speeds up more friction will be created. 4. Once the crate is sliding, how hard do you push to keep it moving at constant velocity? You don’t need to push the crate anymore once its moving. 5. Which is normally greater: static friction or sliding friction on the same object? Between 2 objects, the static friction gets stronger than the sliding friction. 6. How does the force of friction for a sliding object vary with speed? Friction on the sliding object on a flat surface does not vary with speed. 7. Does fluid friction vary with speed? Fluid friction changes with the speed. 8. Which is more fundamental: mass or weight? Which varies with location? Mass is more than weight, because where will change if its in a liquid or on another planet. 9. Fill in the blanks: Shake something to and fro and you’re measuring its____ mass_____ . Lift it against gravity and you’re measuring its____weight_______ . 10. Fill in the blanks: The Standard International unit for mass is the____kilogram________ . The Standard International unit for force is the_____newton______ . 11. What is the approximate weight of a quarter-pound hamburger after it is cooked? About 1 newton 12. What is the weight of a 1-kilogram brick resting on a table? Around 10 newtons. So like2.2 pounds 13. In the string-pull illustration in Figure 4.8, a gradual pull of the lower string results in the top string breaking. Does this occur because of the ball’s weight or its mass? Top string would break because it is being pulled by the balls weight. 14. In the string-pull illustration in Figure 4.8, a sharp jerk on the bottom string results in the bottom string breaking. Does this occur because of the ball’s weight or its mass? Because the ball jerks suddenly the string will break. 15. Is acceleration directly proportional to mass, or is it inversely proportional to mass? Give an example. Acceleration is explained as proportional mass. 16. State Newton’s second law of motion. Newton's second law of motion says that a net force on a body produces an acceleration that is directly proportional to the net force, inversely proportional to the body's mass, and in the same direction as the net force. 17. If we say that one quantity is directly proportional to another quantity, does this mean they are equal to each other? Explain briefly, using mass and weight as an example. No, they are not necessarily equal. The weight of an object is directly proportional to its mass, but the two are not equal. In fact, they are measured in different units. 18. If the net force acting on a sliding block is somehow tripled, what happens to the acceleration? The acceleration is tripled. 19. If the mass of a sliding block is tripled while a constant net force is applied, by how much does the acceleration change? The acceleration is only a third of what it was. 20. If the mass of a sliding block is somehow tripled at the same time the net force on it is tripled, how does the resulting acceleration compare with the original acceleration? The acceleration will be unchanged. 21. How does the direction of acceleration compare with the direction of the net force that produces it? According to Newton's second law, the acceleration and the net force are in the same direction. 22. What is the condition for an object experiencing free fall? When an object experiences free fall, the only force acting on it is the force of gravity. 23. The ratio circumference/diameter for all circles is p. What is the ratio force/mass for freely falling bodies? The ratio of force to mass is g, the magnitude of the acceleration due to gravity. 24. Why doesn’t a heavy object accelerate more than a light object when both are freely falling? The acceleration is calculated as the ratio of force to mass. For all objects in free fall, the ratio is the same, g. 25. What is the net force that acts on a 10-N freely falling object? The net force is 10 N, its weight. 26. What is the net force that acts on a 10-N falling object when it encounters 4 N of air resistance? 10 N of air resistance? 6 N, downward. Zero newtons. 27. What two principal factors affect the force of air resistance on a falling object? The frontal area of the falling object, and its speed, affect the force of air resistance. 28. What is the acceleration of a falling object that has reached its terminal velocity? 0 29. Why does a heavy parachutist fall faster than a lighter parachutist who wears a parachute of the same size? Gravity 30. If two objects of the same size fall through the air at different speeds, which encounters the greater air resistance? The faster object 32. Drop a sheet of paper and a coin at the same time. Which reaches the ground first? Why? Now crumple the paper into a small, tight wad and again drop it with the coin. Explain the difference observed. Will they fall together if dropped from a second-, third-, or fourth story window? Try it and explain your observations. The coin hits the ground before the flat sheet of paper. 34. Drop two balls of different masses from the same height, and, at low speeds, they practically fall together. Will they roll together down the same inclined plane? If each is suspended from an equal length of string, making a pair of pendulums, and displaced through the same angle, will they swing back and forth in unison? Try it and see; then explain using Newton’s laws The net force on each object is proportional to its mass, so the ratio is the same. accelerations are the same 36. Calculate the weight in newtons of a person who has a mass of 50 kg. 500 newtons 38. Calculate the weight in newtons of a 2.5-kg melon. What is its weight in pounds? Around 25 newtons or like 5.5 pounds 40. Susie Small finds that she weighs 300 N. Calculate her mass. Acceleration: a = Fnet/ m I believe 30 kilograms 42. Calculate the acceleration of a 300,000-kg jumbo jet just before takeoff when the thrust on the aircraft is 120,000 N. 0.4 m/s squared 44. In Chapter 3 acceleration is defined as a = Dv Dt . Show that the acceleration of a cart on an inclined plane that gains 6.0 m/s every 1.2 s is 5.0 m/s2 . 5.0m/s squared 46. Knowing that a 1-kg object weighs 10 N, confirm that the acceleration of a 1-kg stone in free fall is 10 m/s2 . 10 m/s squared 48. One pound is the same as 4.45 newtons. What is the weight in pounds of 1 newton? 1 Newton = 0.225 pounds 50. Consider a mass of 1 kg accelerated 1 m/s2 by a force of 1 N. Show that the acceleration would be the same for a force of 2 N acting on 2 kg 1 meter per second squared 52. Alex, who has a mass of 100 kg, is skateboarding at 9.0 m/s when he smacks into a brick wall and comes to a dead stop in 0.2 s. -45 m/s squared 54. Boxes of various masses are on a friction-free, level table. Rank each of the following from greatest to least: D,A,B,C 56. A 100-kg box of tools is in the locations A, B, and C. From greatest to least, rank the_________ a. masses of the 100-kg box of tools. b. weights of the 100-kg box of tools. A.) ABC B.) C,A,B 58. You exert a force on a ball when you toss it upward. How long does that force last after the ball leaves your hand? The force you exert on the ball stops as soon as you lose contact with the ball. 60. If a motorcycle moves with a constant velocity, can you conclude that there is no net force acting on it? Yes How about if it’s moving with constant acceleration? No 62. Which contains more apples: a 1-pound bag of apples on Earth or a 1-pound bag of apples on the Moon? Which contains more apples: a 1-kilogram bag of apples on Earth or a 1-kilogram bag of apples on the Moon? The apples on Earth will weigh more because of gravity. However the mass is unchanged. 64. In an orbiting space vehicle, you are handed two identical boxes, one filled with sand and the other filled with feathers. How can you determine which is which without opening the boxes? Have someone push them towards you then push them back, the one harder to push is the box with sand inside of it. 66. Does the mass of an astronaut change when he or she is visiting the International Space Station? Defend your answer. No, her mass is the same but how gravity affects her changes 68. When a junked car is crushed into a compact cube, does its mass change? Its weight? Explain. The mass and weight do not change. 70. What happens to the weight reading on a scale you stand on when you toss a heavy object upward? As you throw the object, while your hand is in contact with it, the scale reading will increase. 72. What is your own mass in kilograms? Your weight in newtons? 57 killograms and like 560 newtons 74. A crate remains at rest on a factory floor while you push on it with horizontal force F. What is the friction force exerted on the crate by the floor? Explain. The crate remains at rest, so the net force on it is zero. The force of friction exerted by the floor on the crate is equal and opposite to the force you are applying, F. 76. When a car is moving in reverse, backing from a driveway, the driver applies the brakes. In what direction is the car’s acceleration? To the front of the car. 78. Aristotle claimed that the speed of a falling object depends on its weight. We now know that objects in free fall, whatever the gravitational forces on them, undergo the same gain in speed. Why don’t differences in their gravitational forces affect their accelerations? Acceleration is the ration of net force to the mass 80. A race car travels along a raceway at a constant velocity of 200 km/h. What horizontal net force acts on the car? 0 newtons 82. When a coin is tossed upward, what happens to its velocity while ascending? Its acceleration? (Ignore air resistance.) The velocity decreases when the coin is thrown upwards. 84. What is the acceleration of a rock at the top of its trajectory when it has been thrown straight upward? (Is your answer consistent with Newton’s second law?) The rock accelerates downward by 10m/s 86. When your car moves along the highway at constant velocity, the net force on it is zero. Why, then, do you have to keep running your engine? The net force is zero because your car moves at constant velocity. 88. A “shooting star” is usually a grain of sand from outer space that burns up and gives off light as it enters the atmosphere. What exactly causes this burning? Grains of sand moving through the atmosphere burn up because of fluid friction. Heat is generated when friction acts which creates the shooting star. 90. How does the force of gravity on a raindrop compare with the air drag the drop encounters when it falls at constant velocity? It falls with a constant velocity. 92. How does the terminal speed of a parachutist before opening a parachute compared to the terminal speed afterward? Why is there a difference? As the parachute was pulled the jumpers speed drastically decreased. So the parachutist can survive 94. Why does a cat that accidentally falls from the top of a 50-story building hit a safety net below no faster than if it fell from the 20th story? Because the cats terminal speed can be reached by falling 20 feet so the extra 30 feet does not make the cat fall faster. 96. When and if Galileo dropped two balls of the same size but different masses from the top of the Leaning Tower of Pisa, air resistance was not really negligible. Which ball actually struck the ground first? Why? The heavier ball actually strikes the ground first if air resistance is not negligible. 98. In the absence of air resistance, if a ball is thrown vertically upward with a certain initial speed, on returning to its original level it will have the same speed. When air resistance is a factor, will the ball be moving faster, the same, or more slowly than its throwing speed when it gets back to the same level? Why? (Physicists often use the “principle of exaggeration” to help them analyze a problem. Consider the exaggerated case of a feather, not a ball, because the effect of air resistance on the feather is more pronounced and therefore easier to visualize.) When the object goes up it loses speed, when it falls the object has air resistance the speed slows down each second. 100. Make up two multiple-choice questions that would check a classmate’s understanding of the distinction between mass and weight. If a student’s backpack has a mass of 5kg what is the students book bag in newtons? A. 49 Newtons B. 50 Newtons C. 5N D. 43.75N If you weigh 126 pounds on earth, what is your mass on the moon? A. You will be 10 times B. Gravity will not effect you on the moon C. Your mass will not change D. Your mass will not change but your weight will be effected by the moons gravity 102. Discuss whether or not a stick of dynamite contains force. In your discussion make the distinction between contain and exert. The stick of dynamite itself does not have force. 104. An astronaut drops an overhead rock on the Moon. Discuss what force(s) act(s) on the rock as it drops. The moon doesn’t have an atmosphere I believe, so no drag force. The rock should free fall because the moon pulls down on it. 106. Consider a heavy crate resting on the bed of a flatbed truck. When the truck accelerates, the crate also accelerates and remains in place. Identify and discuss the force that accelerates the crate. The friction force between the crate and the truck accelerates the crate forward, keeping it in step with the truck. 108. To pull a wagon across a lawn with constant velocity, you have to exert a steady force. Discuss and reconcile this fact with Newton’s first law, which says that motion with constant velocity requires no force. The wagons constant velocity net force zero. 110. Sketch the path of a ball tossed vertically into the air. (Ignore air resistance.) Draw the ball halfway to the top, at the top, and halfway down to its starting point. Draw a force vector on the ball in all three positions. Are the vectors the same or different in the three locations? Are the accelerations the same or different in the three locations? The force of a vector is probably going to be the same location. Acceleration is also the same at the 3 locations. 112. When you jump vertically off the ground, what is your acceleration when you reach your highest point? Discuss this in light of Newton’s second law. Top you are at 10m/s going toward the ground. At the end of your jump your net force is zero 114. What is the net force acting on a 1-kg ball in free fall? 10N directed toward the ground 116. A discussion partner says that, before the falling ball in the preceding exercise reaches terminal velocity, it gains speed while its acceleration decreases. Do you agree or disagree? Defend your answer. I agree with my partner. 118. Upon which will air resistance be greater: a sheet of paper falling or the same sheet of paper wadded into a ball falling at a faster terminal speed? (Careful!) The wadded up ball of paper will have less air resistance because the mass is changed when you reshape the original shape. 120. A 400-kg bear grasping a vertical tree slides down at constant velocity. What is the friction force that acts on the bear? Discuss how “constant velocity” is the key to your answer. 4000 newtons. The constant velocity of the bears frictional force is a zero net force because the acceleration is also Zero.