PHYSICS Dynamics. gravity weight air resistance inertia friction normal force

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1 LESSON OBJECTIVES Students will be able to... PHYSICS Dynamics define force describe Newton s First Law of Motion using inertia identify the criteria of balanced and unbalanced forces distinguish among net, applied and oppositional forces describe Newton s Second Law of Motion with respect to an unbalanced force understand that force is the cause of acceleration understand that gravity is an attractive force between two masses differentiate between mass and weight identify forces acting on a body that contribute to or hinder its motion apply vector resolution to a force acting perpendicularly identify characteristics of friction and normal force describe Newton s Third Law of Motion SKILLS Students will be able to... use appropriate metric units for given measurements use steps for solving physics problems draw free body diagrams solve problems using mass, acceleration and force, including gravity conduct experiments relating to force and motion collect and display experiment data develop models of motion and force VOCABULARY: acceleration force mass balanced force unbalanced force net force gravity weight air resistance inertia friction normal force applied force static friction coefficient of friction Newton (N) derived unit HOMEWORK ASSIGNMENTS Newton's First Law of Motion #1: Newton's First Law, reading and homework page 1 #2: Inertia and Mass, reading and homework page 2 #3: State of Motion, reading #4: Balanced and Unbalanced Forces, reading and homework pages 3-4 1

2 Force and Its Representation #5: The Meaning of Force, reading #6: Types of Forces, reading and homework pages 5-9 #7: Drawing Free-Body Diagrams, reading and homework page 10 #8: Determining the Net Force, reading and homework page Newton's Second Law of Motion #9: Newton's Second Law, reading and homework page 13 #10: The Big Misconception, reading #11: Finding Acceleration, reading and homework page #12: Finding Individual Forces, reading and homework page 18 Newton's Third Law of Motion #13: Newton's Third Law, reading homework page #14: Identifying Action and Reaction Force Pairs, reading and homework page CONTENT OUTLINE I. Newton s First Law of Motion A. Mass and Inertia B. Combining Forces 1. Balanced Forces 2. Unbalanced Forces II. Nature of Force A. Definition B. Types 1. Friction a. Factors Affecting Gravity b. Static and Dynamic Friction 2. Gravity a. Factors Affecting Gravity i. mass ii. distance b. Weight vs. Mass 3. Other C. Free Body Diagrams D. Net Force III. Newton s Second Law of Motion A. Calculations B. Using Free Body Diagrams IV. Newton s Third Law of Motion 2

3 Newton s First Law of Motion 1. When serving in volleyball, the act of striking the ball is an example of applying a. a pushing force c. no force at all b. a pulling force d. both a pushing and pulling force 2. A person weighs less at the top of a mountain than at the base of a mountain because a. a person has less mass at the top of a mountain b. a person has more mass at the top of a mountain c. a person is farther from the center of Earth when at the top of a mountain d. there is less air pressure at the top of the mountain 3. The force of 1 pound is equal to 4.5 newtons and the force of 1 kilogram is equal to 9.8 newtons. Which force is greater? a. 10 pounds b. 10 kg c. 10 newtons d. 50 newtons A thrown baseball will move in a straight line at a constant speed (called uniform motion) until another force affects it. The application of other forces will cause the ball to eventually slow and stop. 4. What force does arrow 1 represent? 5. What force does arrow 2 represent? 6. Name another force that could also slow the ball and/or stop its motion. Roberto threw a softball up into the air. It reaches a height of 9 meters and then started to come down. He then threw the ball to a height of 12 meters. 7. What force pulled the ball down? 8. What is the most likely reason that the ball went higher on Roberto s second throw? 9. What two conditions are necessary for uniform motion? 10. What is a force? 11. How is a force described? 3

4 12. In a diagram, one force arrow is longer than the other arrow. What can you tell about the forces? 13. How can you find the net force if two forces act in opposite directions? 14. How do balanced forces acting on an object affect its motion? How do unbalanced forces acting on an object affect its motion? 15. You exert a force of 120 N on a desk. Your friend exerts a force of 150 N in the same direction. What net force do you and your friend exert on the desk? 16. Terrance helped his dad push their car out of the garage to repair it. He pushed as hard as he could, but the car only moved very slowly. This example demonstrates the principle that a. A body at rest remains at rest unless a force affects it. b. A body in motion remains in motion unless a force affects it c. A large mass requires a large force to move it d. Every action has an equal but opposite reaction 17. On a breezy day, Esther pushed her model sailboat across the still water in a pond. According to Newton s first law, the sailboat should have continued across the pond and landed at Point A. However, it landed at Point B. The most likely reason for this was that a. the force of the wind changed the sailboat s direction of motion b. fish swimming near the sailboat changed its direction of motion c. the force of the moving water changed the sailboat s direction of motion d. the sailboat accelerated across the pond 18. What is the relationship between the mass and inertia of an object? a. the greater the mass, the greater the inertia b. the greater the mass, the less the inertia c. the less the mass, the greater the inertia d. the mass and the inertia are not related 19. Which of the following objects would need the greatest force to be stopped? a. a baseball moving at 80 km/hr c. a train moving at 80 km/hr b. a car moving at 80 km/hr d. an arrow moving at 80 km/hr 4

5 Gail and Jared set up an experiment that they read about in their science textbook which used a glass, several coins (penny, nickel, and quarter), and an index card. The students flicked the card from under each of the coins to see what would happen. 20. When they flicked the card from under each of the coins, a. the coins all stayed with the card and fell on the table b. the coins all dropped into the glass c. the quarter fell into the glass, and the other two coins fell on the table d. the penny fell into the glass, and the other two coins fell on the table 21. This experiment indicated that an object at rest will remain at rest and will resist being put into motion. What term best expresses what this experiment demonstrated? a. differences in mass c. inertia b. acceleration d. differences in motion 22. The quarter has the greatest mass of the three coins. Therefore, it would have the greatest a. resistance to change in motion c. change in direction b. acceleration d. distance to fall 23. Explain why a lead ball 5 cm in diameter has more inertia than a wooden ball 5 cm in diameter. 24. What does Newton s 1st Law say? 25. If you hold a coin above (and a tiny bit to the side of) your head while standing in a bus that is not moving, the coin will land at our feet when you drop it. Where will it land if the bus is moving in a straight line at constant speed? Why? 26. Your brain is not attached by any tissue to your skull, it simply floats inside. A concussion happens when your brain hits the inside of your skull and gets bruised or damaged. Use Newton s first law to explain what happens when you get a concussion by running into a wall. Friction and Gravity 27. What factors affect the friction force between two surfaces? 28. What is the law of universal gravitation? 29. How do mass and distance affect the gravitational attraction between objects? 5

6 30. How would your weight change on the surface of an Earth-sized planet whose mass was greater than Earth s? Why? 31. Why does an object accelerate when it falls toward Earth s surface? 32. How does the mass of an object affect its acceleration during free fall? 33. Which of the four different balls listed would require the greatest force to move? a. golf ball c. bowling ball b. baseball d. basketball 34. The mass of the stature of liberty would be a. greatest in NYC b. greatest at the north pole c. greatest at the moon d. the same in all locations 6

7 35. A woman weighs 580 N. What is her mass? 36. Find the weight of a 2000 kg elephant. 37. An astronaut has a mass of 85 kg. Calculate his weight on Earth and on the Earth s moon (gmoon = 1.6 m/s 2 ). Does his mass change when he goes from Earth to its moon? 38. Calculate the weight of a 4.5 kg rabbit. 39. At the surface of Mars the acceleration due to gravity is 3.8 m/s 2. A book weighs 34 N at the surface of the Earth. What is its mass on the earth s surface? What are its mass and weight on Mars s surface? 40. A boy weighs 270 N. What is his mass? 41. Find the weight of a 60 kg table. 42. A Martian weighs 17 N on the surface of Mars. Calculate his weight on Earth and on the Earth s moon. Does his mass change along the flight from Mars to the Moon to the Earth? The acceleration due to gravity on Mars is 3.8 m/s 2 and the acceleration due to gravity on the Moon is 1.6 m/s Find the weight of a 2 g computer chip. 44. What is the mass of a 330 N television? 45. The mars rover, Spirit has a mass of 836 kg. Calculate its weight on Earth and on Mars. The acceleration due to gravity on Mars is 3.8 m/s 2 Newton s Second Law 46. How could you keep an object s acceleration the same if the force acting on the object were doubled? 47. Using what you know about Newton s second law, explain why a car with a large mass might use more fuel than a car with a smaller mass. Assume both cars drive the same distance. 48. Find the force it would take to accelerate an 800-kg car at a rate of 5 m/s What is the net force acting on a 0.15-kg hockey puck accelerating at a rate of 12 m/s 2? 7

8 50. How much force is needed to accelerate a 68 kilogram-skier at a rate of 1.2 m/sec2? 51. What is the mass of an object that requires a force of 30 N to accelerate at a rate of 5 m/sec 2? 52. What is the force on a 1,000 kilogram-elevator that is falling freely under the acceleration of gravity only? 53. What is the mass of an object that needs a force of 4,500 N to accelerate it at a rate of 5 m/sec 2? 54. What is the acceleration of a 6.4 kilogram bowling ball if a force of 12 N is applied to it? 55. According to Newton s second law, an increase in force on a given mass will cause: a. a decrease in acceleration c. a decrease in the mass b. an increase in acceleration d. an increase in the mas 56. A rocket traveling in space increases its velocity from 32,000 km/hr to 40,000 km/hr. To do this, the rocket must a. increase its mass c. increase the force acting on it b. decrease its mass d. decrease the force acting on it 57. A 0.40 kg toy car moves at constant acceleration of 2.3 m/s 2. Determine the net applied force that is responsible for that acceleration. 58. If a net horizontal force of 175 N is applied to a bike whose mass is 43 kg what acceleration is produced? 59. What average net force is required to stop a 7 kg shopping cart in 2 s if it s initially traveling at 3.5 m/s? 60. A 7.5 kg cannon ball leaves a canon with a speed of 185 m/s. Find the average net force applied to the ball if the cannon muzzle is 3.6 m long. 61. A wooden block is pulled at a constant acceleration of 1.4 m/s2. Find the net applied force on the block if its mass is 0.6 kg. 62. A 95 N net force is applied to an ice block with a mass of 24 kg. Find the acceleration of the block if it moves on a smooth horizontal surface. 63. A net force of 345 N accelerates a boy on a sled at 3.2 m/s2. What is the combined mass of the sled and boy? 64. What average net force is required to stop an 8500 kg truck in 10 s if it s initially traveling at 20 m/s? 65. What average net force is required to accelerate a 9.5 g bullet from rest to 650 m/s over a distance of 0.85 m along the barrel of a rifle? 8

9 66. A physics student pushed a 50 kg load across the floor, accelerating it at a rate of 1.5 m/s2. How much force did she apply? 67. A 10,000 N net force is accelerating a car at a rate of 5.5 m/s2. What is the car s mass? 68. A boy pedals his bicycle with a net horizontal force of 235 N. If the total mass of the boy and the bike is 40 kg, how much are they accelerating? 69. A 45 kg swimmer starting from rest can develop a maximum speed of 12 m/s over a distance of 20 m. How much net force must be applied to do this? 70. A net force of 3000 N is accelerating a 1200 kg elevator upward. If the elevator starts from rest, how long will it take to travel up 15 m? 71. A 57 kg paratrooper falls through the air. How much force is pulling him down? 72. A net force of 34 N is applied to accelerate an object at a rate of 2.5 m/s2. What is the mass of the object? 73. A runner exerts a net force of 225 N to accelerate at a rate of 3.0 m/s2. What is the runner s mass? 74. What average net force is required to stop a 4 kg bowling ball in 0.5 s if it s initially traveling at 10 m/s? 75. A hockey puck with a mass of 0.18 kg is at rest on the horizontal frictionless surface of the rink. A player applies a horizontal force of 0.5 N to the puck. Find the speed and the traveled distance 5 s later. 9

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12 Newton s Third Law The third law of motion states that for every action there is an equal and opposite reaction. A simple demonstration of blowing up a balloon and letting it go shows how this law works. When the air is released from an opening in the balloon, the balloon moves in the opposite direction. Other actions that produce and equal and opposite reaction force include kicking a soccer ball, waling, and hot gas shooting out of a rocket engine. 76. According to Newton s third law of motion, how are action and reaction forces related? 77. What would happen if you tried to catch a ball when you were standing on roller skates? 78. A CO2 cartridge was mounted on the top of a toy car with the nozzle of the cartridge pointing toward the rear of the car. When the compressed gas was released from the cartridge, the toy car moved 6 meters across the room and stopped. What can be changed so that the car will travel a shorter distance? 79. The third law of motion states that for every action there is an equal and opposite reaction. A simple demonstration of blowing up a balloon and letting it go shows how this law works. When the air is released from an opening in the balloon, the balloon moves in the opposite direction. Other actions that produce and equal and opposite reaction force include kicking a soccer ball, waling, and hot gas shooting out of a rocket engine. 80. According to Newton s third law of motion, how are action and reaction forces related? 12

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16 Free body diagrams Coefficient of Friction: Symbol FN Meaning an unit FW Ffr Fa µ FN=Fw Ffr = µfn 81. A box sits at rest on a tabletop. Draw and clearly label all the forces acting on the box; compare their magnitudes and directions. 82. A wooden block moves at a constant speed on a rough horizontal surface. Draw a freebody diagram clearly showing all the forces applied to the block; compare their magnitudes and directions. 83. A crane lifts a load at a constant speed. Draw a free-body diagram for the load and compare the magnitudes and directions of the all forces. 84. A crate is accelerated at a constant rate along a rough horizontal floor. Draw a freebody diagram for the crate and compare all the forces exerted on the crate. 85. A hockey puck slides on a rough horizontal surface. Draw a free-body diagram for the puck and compare the magnitudes and the directions of all the forces exerted on it. 86. A boy pulls a sled horizontally at a constant speed by holding a rope that is connected to the sled. a. Show all the forces exerted on the sled (do not ignore friction); b. Show all the forces exerted on the boy (do not ignore friction); c. Show all the forces acting on the rope; d. Use Newton s Law to explain the directions and the magnitudes of all the forces; compare action and reaction. 87. A hockey puck has a coefficient of kinetic friction of μk =.10. I f the puck feels a normal force (FN) of 5.0 N, what is the frictional force that acts on the puck? 88. Suppose a 10.0 N force is applied to the side of a 4.0 kg block that is sitting on a table. The block experiences a frictional force against the force that is applied. a. Draw a force diagram for the block. b. What is the weight of the block (Fw)? c. What is the normal force on the block (FN)? d. I f the coefficient of friction is μ =.20, w hat is the frictional force on the block (Ff)? e. What is the net force on the block? f. What is the acceleration of the block from the net force? 16

17 89. For the block in Problem 2, what would be the minimum value of the coefficient of static friction in order for the block to remain motionless? 90. A block weighing N is moved at a constant speed over a horizontal surface by a force of 50.0 N applied parallel to the surface. What does the constant speed tell you about the forces acting on the block? a. Draw a force diagram for the block. b. What is the coefficient of kinetic friction (μk)? c. What is the mass of the block? d. What w ould be the acceleration of the block if μ = 0.0? 91. A 100. N force is applied to the side of a crate resting on a level floor. The crate has a mass of 50. kg. If the coefficient of static friction is μ =.25, is the 100. N force enough to make the crate accelerate? Explain why or why not. (As always, start with a force diagram) 92. The coefficient of kinetic friction between an object and the surface upon which it is sliding is The weight of the object is 20N. What is the force of friction? 93. The force of friction between an object and the surface upon which it is sliding is 12N. The weight of the object is 20N. What is the coefficient of kinetic friction? 94. The coefficient of kinetic friction between an object and the surface upon which it is sliding is The mass of the object is 3.2 kg. What is the force of friction? 95. The force of friction between an object and the surface upon which it is sliding is 15N. The mass of the object is 20kg. What is the coefficient of kinetic friction? 96. The coefficient of kinetic friction between an object and the surface upon which it is sliding is The weight of the object is 80N. What is the force of friction? 97. The coefficient of kinetic friction between an object and the surface upon which it is sliding is 0.6. The mass of the object is 12 kg. What is the force of friction? 98. The force of friction between an object and the surface upon which it is sliding is 12N and the coefficient of friction between them is What is the weight of the object? 99. The force of friction between an object and the surface upon which it is sliding is 250N and the coefficient of friction between them is What is the mass of the object? 100. The force of friction between an object and the surface upon which it is sliding is 36N. The weight of the object is 85N. What is the coefficient of kinetic friction? 101. The coefficient of kinetic friction between an object and the surface upon which it is sliding is The mass of the object is 8.0 kg. What is the force of friction? 102. The force of friction between an object and the surface upon which it is sliding is 46N and the coefficient of friction between them is What is the weight of the object? 17

18 103. The force of friction between an object and the surface upon which it is sliding is 360N. The mass of the object is 95kg. What is the coefficient of kinetic friction? 104. The force of friction between an object and the surface upon which it is sliding is 126 N and the coefficient of friction between them is What is the mass of the object? 105. The force of friction between an object and the surface upon which it is sliding is 12 N and the coefficient of friction between them is What is the weight of the object? 106. The coefficient of kinetic friction between an object and the surface upon which it is sliding is The mass of the object is 16kg. What is the force of friction? 107. The force of friction between an object and the surface upon which it is sliding is 3.5N. The mass of the object is 4 kg. What is the coefficient of kinetic friction? 108. The force of friction between an object and the surface upon which it is sliding is 100 N and the coefficient of friction between them is What is the mass of the object? 109. A stationary 15 kg object is located on a table near the surface of the earth. The coefficient of static friction between the surfaces is 0.40 and the coefficient of kinetic friction is A horizontal force of 20 N is applied to the object. a. Draw a free body diagram with the forces to scale, b. Determine the force of friction, c. Determine the acceleration of the object, d. Now a horizontal force of 40 N is applied to the object. e. Draw a free body diagram with the forces to scale. f. Determine the force of friction. g. Determine the acceleration of the object. h. Now a horizontal force of 60 N is applied to the object. e. Draw a free body diagram with the forces to scale. j. Determine the force of friction. k. Determine the acceleration of the object. l. Now a horizontal force of 100 N is applied to the object. m. Draw a free body diagram with the forces to scale. n. Determine the force of friction. o. Determine the acceleration of the object A stationary 250 kg object is located on a table near the surface of the earth. The coefficient of static friction between the surfaces is 0.30 and the coefficient of kinetic friction is A horizontal force of 300 N is applied to the object. a. Draw a free body diagram with the forces to scale. b. Determine the force of friction. c. Determine the acceleration of the object? d. A horizontal force of 1500 N is applied to the object. e. Draw a free body diagram with the forces to scale. f. Determine the force of friction. g. Determine the acceleration of the object. 18

19 111. A stationary 2.0 kg object is located on a table near the surface of the earth. The coefficient of static friction between the surfaces is 0.80 and the coefficient of kinetic friction is A horizontal force of 5 N is applied to the object. Repeat a-c for the application of a 20 N force. a. Draw a free body diagram with the forces to scale. b. Determine the force of friction. c. Determine the acceleration of the object A 1500 kg elevator moves up and down on a cable. Calculate the tension in the cable for the following cases: a. The elevator moves at constant speed upward b. The elevator moves at constant speed downward c. The elevator accelerates upward at a constant rate of 1.2 m/s2 d. The elevator accelerates downward at a constant rate of 1.2 m/s A crane accelerates a 175 kg load is upward. The tension in the cable is 2000 N. Find the magnitude and direction of the elevator s acceleration A 65 kg woman is inside an elevator. Calculate her apparent weight for the following cases: a. The elevator moves at constant speed upward b. The elevator moves at constant speed downward c. The elevator accelerates upward at a constant rate of 2.4 m/s2 d. The elevator accelerates downward at a constant rate of 2.4 m/s An 800 N man stands on a scale in a motionless elevator. When the elevator begins to move, the scale reads 650 N. Find the magnitude and direction of the elevator s acceleration A train with a mass of kg increases its speed from 10 m/s to 25 m/s in 20 seconds. Assume that the acceleration is constant and that you can neglect friction. a. Find the acceleration of the train b. Find the distance traveled during this 20 s? c. Draw a free- body diagram for the train; d. Find the average net force supplied by the locomotive A 150 kg motorcycle starts from rest and accelerates at a constant rate along a distance of 350m. The applied force is 250 N and the coefficient of kinetic friction is a. Draw a free-body diagram for the motorcycle showing all applied forces to scale. Next to that diagram show the direction of the acceleration; b. Find the net force applied to the motorcycle; c. Find the acceleration of the motorcycle; d. What is its speed at the end of 350 m? e. Find the elapsed time of this acceleration. 19

20 Directions: For each of the following, write the letter of the choice that best completes the sentence When you exert a force on an object it exerts force back on you. a. a stronger b. the same 119. When volleyball players jump into the air, the primary force acting to make them land back on the ground is. a. mass b. gravity 120. Forces always act in. a. pairs b. singles 121. In a game of tug-o-war, the team that wins has exerted a greater force. a. unbalanced b. mass 122. When you are pushing on a large door, friction keeps you from sliding backwards. a. unbalanced b. static 123. is determined by gravity. a. weight b. mass 124. A component of inertia is. a. mass b. friction 125. friction causes a car tire to turn on the road. a. Static b. Rolling 126. friction keeps an object from moving when a force is applied. a. Static b. Sliding 127. If the same force is applied to two different objects, the one with the mass has a smaller acceleration. a. larger b. smaller 20

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