Forces Newton s Three Laws of Motion Newton s First Law of Motion The Law of Inertia

Transcription

1 Physics R Date: 1. A 2009 Ford Mustang convertible is travelling at constant velocity on Interstate 95 south from Philadelphia to Wilmington Delaware. It passes the 120 mile marker at exactly 1:00pm and the 130 mile marker at 1:08pm. What is the acceleration of the car? Forces Newton s Three Laws of Motion Newton s First Law of Motion The Law of Inertia Inertia 2. Which person has the greatest inertia? (1) a 110-kg wrestler resting on a mat (3) a 90-kg man walking at 2 m/s (2) a 70-kg long-distance runner traveling at 5 m/s (4) a 50-kg girl sprinting at 10 m/s Newton s Second Law of Motion Change in straight line motion is Objects with more inertia ( ) resist change in straight line motion ( ) Therefore is proportional to A force is Net force is 1

2 3. Mr. Vigneaux pushes on a box with a force of 10 N, while Mr. Evans pushes the other direction with a force of 7 N. What is the net force on the box? 4. A 1200kg car is accelerated at a rate of 4 m/s/s. What is the net force acting on the car? 5. A train engine with a mass of 5000 kg is pushed with a force of 12,000 N to the East. Friction provides a force of 2,000 N to the West. What is the net force on the train? What is the acceleration of the train? 6. The following forces act on a box of mass m. The box accelerates at a rate of 6 m/s/s. What is the mass, m, of the box? Newton s Third Law of Motion I push down on the desk with a force of 10 N The desk pushes up on me with a force of How does an airplane propeller push an airplane forward? Would an airplane propeller work in the vacuum of space? 7. A 100 kg ice skater pushes on a 50 kg ice skater with a force of 200 N. a. What is the force on the 100 kg ice skater? b. What is the acceleration of the 50 kg ice skater? c. What is the acceleration of the 100 kg ice skater? 2

3 SPACE LEFT BLANK FOR CONCEPTUAL PHYSICS 3 RD LAW PAGE 3

4 8. A 50kg goat travelling at 10 m/s slows to rest over 25 m. What is the net force on this goat? (Hint: Find acceleration first) Four fundamental forces All interactions come from these four forces Weight vs. Mass Weight Gravity pulls down twice as hard on a 2kg mass than a 1 kg mass. Why do they free fall with the same acceleration? 9. What is the weight of a 2.5 kg textbook? 10. A student weighs 600 N. What is his mass? 11. An object has a weight of 100 N on Earth. What is the weight of the same object on the Moon, where acceleration due to gravity is 1.6 m/s/s? 4

5 Other forces Normal Force Friction Tension (and more) Normal Force What is the weight of a 1 kg mass, falling freely to the Earth? What is its acceleration? What is the weight of a 1 kg mass, sitting on the top of a table? What is its acceleration? Explain this behavior Normal Force 12. A 10 kg dog sits on the floor. What is the normal force pushing up on him? 13. A 600 N student sits at his desk. What is the normal force pushing up on him? 14. A lift provides a normal force of 200 N to a 12 kg object. What is the weight of the object? What is the net force on the object? What is the acceleration of the object? 5

6 Newton s Laws Practice 15 How does the inertia of a 2 kilogram rock compare to the inertia of a 1 kilogram rock? (explain) 16 If you were in a spaceship and fired a cannonball into frictionless space, how much force would have to be exerted on the ball to keep it going, explain. 17 Which has more inertia, a bullet traveling at 100 m/s or a bowling ball slowly rolling across the floor, explain. 18 If an elephant were chasing you, its enormous mass would be very threatening. But if you zigzagged, the elephant s mass would be to your advantage. Why? 19 Forces of 10 N and 15 N in the same direction act on an object. What is the net force on the object? 20 Suppose a cart is being moved by a certain net force. If the net force is doubled, by how much does the cart s acceleration change? (explain or show example) 21 Suppose a cart is being moved by a certain net force. If a load is dumped into the cart so its mass is doubled, by how much does the acceleration change? (explain or show example) 22 How much net force acts on a 20,000 kg rocket that accelerates at 1 m/s 2 6

7 23 If an object has no acceleration, can you conclude that no forces are exerted on it? Explain. 24 A rocket fired from its launching pad not only picks up speed, but its acceleration also increases significantly as firing continues, why is this so? (Hint: about 90% of the mass of a newly launched rocket is fuel.) 25 A horizontal force of 100 N is required to push a crate across a factory floor at a constant speed. What is the net force acting on the crate? What is the force of friction acting on the crate, justify your answers. 26 The force of gravity is twice as great on a 2kg rock as on a 1 kg rock. Why does the 2 kg rock not fall with twice the acceleration? 27 A 1.5 kg book sits on a bookshelf. Calculate the normal force acting on the book. 28 An unfortunate bug splatters against the windshield of a moving car. Compared to the force of the car on the bug, the force of the bug on the car is: 29 A tennis ball and a solid steel ball the same size are dropped at the same time. In the absence of air resistance, which ball has the greater acceleration? 7

8 30. What is the mass of a person who weighs 100 N on the moon? (g = 1.6 m/s/s on the moon) What would that person's weight be on Earth? When an object is in equilibrium: Equilibrium What does unbalanced force mean? Equilibrant Concurrently 31. Are the following objects in equilibrium? If not, sketch and calculate the equilibrant. 32. A 75 kg skydiver falls with constant velocity. What is his weight? What is the net force on him? Is he in equilibrium? What is the force from air resistance? 33. A 5 kg block is being dragged across the floor in a straight line with a constant speed of 10 m/s. If the pulling force is 25 N, what is the force of friction acting on the block? 8

9 Free Body Diagrams - All objects are represented as a point 34. A 1.5 kg box hangs by a string. What is the net force on the box? Is the object in equilibrium? What is the force of gravity? What is the tension force in the string? Draw a free body diagram to the right 35. A traffic light is held up by 2 cables What is the net force on the light? Is the object in equilibrium? Draw a free body diagram to the right 36. A 1200 kg car is travelling on the highway at 30 m/s. The engine provides 1200 N of force propelling it forwards. Drag on the car provides 2000 N of force pushing it back. Is the car in equilibrium? Create a free body diagram showing force from the engine (F engine ), force from drag (F drag ), weight(f g ), and normal force (F N ). What is the net force on the car? What is the acceleration of the car? 37. A little boy pulls his 5 kg wagon with a force of 50 N. Friction provides a force of 20 N in the opposite direction. Is the wagon in equilibrium? Create a free body diagram, showing 4 forces acting on the wagon (1 being a tension force applied by the boy) Is the wagon in equilibrium? What is the net force on the wagon? What is the weight of the wagon? What is the normal force acting on the wagon? 9

10 38. A force of 50 Newtons is used to drag a 10 kg box across a horizontal table. If a frictional force of 15 Newtons is present on the box, a. calculate the unbalanced (net) force on the box. b. Calculate the acceleration of the box c. Calculate the force of gravity acting on the box 39. A 2 kg mass moving along a horizontal surface at 10 m/s is slowed by a 5 N force of friction. a. Calculate the net force. b. Calculate the acceleration. 40. A 20 N box rests on a table top. What is the normal force acting on the box? 41. A 5 kg mass is falling freely. What is the normal force acting on the mass? 42. A boy pulls a 1 kg toy duck with constant velocity. He pulls with a force of 2N. What is the acceleration of the duck? Normal Force & Gravity Don t confuse Normal force with Net force! In 95% of physics problems with an object resting on a surface, F N = F g, why? What about the other 5%? Notable examples when F N F g : 43. A 20 N object rests on a table top. Calculate the normal force and net force. 44. A 0.5 kg ball falls freely from rest. Calculate the normal force and net force acting on the ball 10

11 45. Suppose you (60 kg student) are on an elevator. The elevator accelerates upward at 2 m/s 2. a. What do you feel (heavier or lighter)? b. Draw a free body diagram c. What is the net force on you? d. What is your weight? e. What is the normal force? 46. Now the elevator slows to a stop at a rate of 1 m/s 2. a. Draw a free body diagram b. What is the net force on you? c. What is your weight? d. What is the normal force? What does a scale measure? 47. An elevator is traveling up at 10 m/s, and slows to a stop in 4.3 seconds. a. What is the acceleration of the elevator? In what direction? b. A 60 kg man is standing on a scale in the elevator. What is his weight? c. What is the net force acting on the man? In what direction? d. What will the scale read (in N)? 48. A 50 kilogram crate is pushed along a flat, frictionless surface with a force of 100 Newtons. Sketch this object. Include all forces with labels and quantities. What rate of acceleration will the crate experience? 11

12 49. What is the net force acting on a 3000 Newton rocket if its engine produces an upward thrust of 4500 Newtons? 50. A 1.5-kilogram lab cart is accelerated uniformly from rest to a speed of 2.0 meters per second in 0.50 second. What is the magnitude of the force producing this acceleration? 51. A force of 25 newtons east and a force of 25 newtons west act concurrently on a 5-kilogram cart. What is the acceleration of the cart? 52. Which body is in equilibrium? 1. a satellite orbiting Earth in a circular orbit 2. a ball falling freely toward the surface of Earth 3. a car moving with a constant speed along a straight, level road 4. a projectile at the highest point in its trajectory Forces Quick Review F net means 2 things: What does F g mean? How do you calculate F g? What does F N mean? F N usually is equal to F g because What does equilibrium mean? Describe the motion of an object in equilibrium Review Checklist Explain the relationship between mass and inertia 1. Which of the following has the greatest inertia? (a) 10 kg object (c) 20 kg object (b) 15 kg object (d) 25 kg object Explain the relationship between net force, mass, and acceleration. 2. A 20 newton net force acts on a 5.0 kilogram object. What rate of acceleration will this object experience? 12

13 3. A 10 kg bowling ball is traveling with a constant velocity of 15 m/s. How much force is necessary to maintain this velocity? [Neglect Friction] 4. A 10 Newton force South and a 5 Newton force North act on a 2 kg box. What is the magnitude of the acceleration of the box? 5. A 10 Newton force South and a 5 Newton force West act on a 2 kg box. What is the magnitude of the acceleration of the box? Explain the relationship between mass and weight. 6. What is the weight of a 5.0 kilogram object when it is near the surface of the Earth? 7. The acceleration due to gravity on Planet X is 6.0 m/s 2. What is the weight of a 4.0 kilogram object on Planet X? 8. A 2.5 kilogram object is transported from Earth to the Moon. When the object arrives on the Moon its weight is (a) compared to that on Earth while its mass is (b) compared to that on Earth. Explain the relationship between action/reaction pairs 9. A boy pushes a cart across the floor with a force of 10 newtons. The cart pushes on the boy with a force of Calculate Normal force acting on an object 10. What is the normal force acting on a 20 kg object sitting on a table? 11. A 2000 kg car is traveling on the highway at 30 m/s. What is the normal force acting on the car? 12. A 12 N book sits on a bookshelf. Calculate the normal force acting on the book? 13. An apple falls from a tree. Calculate the normal force acting on the apple while it is in free fall? 14. A 600 N person stands on an elevator. Calculate his normal force: a. When the elevator is at rest b. When the elevator begins to accelerate upwards at 1.5 m/s 2 c. When the elevator is traveling upwards at a constant 6 m/s d. When the elevator accelerates downwards at 3 m/s 2. Describe the motion and state of forces of an object in equilibrium 15. Describe the two possible states of motion for an object at equilibrium. 16. A 1500 kg car travels 600 meters with a constant velocity of 30 m/s. It does so in 20 seconds. What is the acceleration of the car? What is the net force of the car? 17. A 20 N force North and a 20 N force East act concurrently on an object. What additional equilibrant force would be necessary to bring the object to equilibrium? In what direction? 18. A 50 kg skydiver falls with constant velocity. Find the force of air resistance acting on the skydiver 13

14 19. Draw/calculate the equilibrant force necessary for each. If none, say none. 20. Which of the following is in equilibrium? a. A car traveling down a straight road at 45 mph b. An object in free fall c. A soccer ball at rest on a field d. A soccer ball following a projectile path e. A drag racer accelerating at a constant rate f. A wagon being pulled with a constant velocity Draw a free body diagram, solve for unknown forces 21. A man pulls a 10 kg box with a horizontal force of 150 N. 50 N of friction act on the box. Make a free body diagram, solve for all forces acting on the box, and determine the acceleration of the box. 22. One Saturday evening you are out at a physics study session and your friend passes out from too much physics fun. You tie a rope to your 980 N friend and pull him accelerating him at 0.7 m/s 2. Make a free body diagram, solve for all forces acting on your friend, and determine the net force. 1. d 2. 4 m/s N m/s m/s N N 8. (a) Less, (b) the same N N N N N 14. (a) 600 N, (b) 692 N, (c) 600 N, (d) 416 N 15. At rest, constant velocity m/s 2, 0 N N Southwest N 19. (1) 5 N up, (2) none, (3) 3N up, (4) 2N left + 2 N down (2.8 N resultant) 20. a, c, f 21. a = 10 m/s Fnet = 70 N 14