HOW ABOUT THEM APPLES?

Transcription

1 Physics 432 Newton s Laws Name Date I. Mass vs. Weight Answer the following questions true or false. F_1. A Newton is a unit of mass. HOW ABOUT THEM APPLES? T 2. Weight is a measure of gravitational force. F 3. Mass is the same as weight. F_4. Changing gravitational acceleration will change mass. F_5. On the moon, your weight would be the same as on the earth. T 6. Gravitational force depends on mass and the distance between two objects. T_7. As mass increases, so does weight. T_8. An unbalanced force will always cause a change in motion. T 9. A net force will occur when there are two unbalanced forces. T 10. The gravitational force acting on a 10-kg object is twice as great as that acting on a 5-kg object. 11. A student has a mass of 68 kg. What is the student s weight? M = 68 kg. g= 10 m/s 2 W =? W =mg = 68 kg * 10 m/s 2 = 680 N 12. What is the student s mass if she has a gravitational force of 150 N on earth? F= 150 N g =10 m/s 2 m=? F = ma m = F/a = 150 N/ 10 m/s 2 = 15 kg 13. A mass of 56 kg has a weight of 504 N on top of a mountain. What is the value of g? m= 56 kg. W = 504 N g =? W = mg g= W/m = 504 N/56 kg. = 9m/s 2

2 II. Newton s Second Law of Motion 14. An object of mass 70 kg is accelerated by a force of 20 N. What is its acceleration? m= 70 kg. F= 20 N a =? F = ma a= F/m = 20 N/70 kg =.28 m/s 2 a= 25 m/s 2 F = 500 N m =? F= ma m = F/a = 500 N/ 25 m/s 2 = 20 kg 15. What is the mass of an object that is accelerated at 25 m/s 2 that is pushed with a force of 500 N? 16. A 75-kg person traveling at a constant velocity on ice skates experiences a force of 40 N. What is the person s acceleration? 17. A stone weighs 4.5 N. What force must be applied to make it accelerate upwards at 3.0 m/s 2? m = 75 kg. F= 40 N a =? F= ma a= F/m = 40 N/75 kg. = 0.53 m/s 2 Draw a FBD W= 4.5 N a = 10 m/s 2 m=? W=mg m= W/g = 4.5 N/ 10 m/s 2 =.45 kg F net = ma =.45 kg * 3 m/s 2 = 1.35 N F net = F up -W F up = F net +W = 1.35 N N = 5.85 N 18. A constant force acts on a 5 kg object and reduces its velocity from 7 m/s to 3 m/s in a time of 3 seconds. Find the amount of force acting on it. m = 5 kg vi = 7 m/s vf=3 m/s t = 3 sec a = vf-vi/t = (7 3 m/s)/3 sec = 1.33 m/s 2 F = ma = 5 kg * 1.33 m/s 2 = 6.65 N m= 500 kg. vi = 0 m/s t = 3sec d = 200 m d = v i t + ½ at 2 = ½ at 2 a = 2d/ t 2 = 2 * 200 m/ (3s) 2 =44.4 m/s A speedboat has a mass of 500 kg. It starts from rest and travels 200 m in 3 seconds. The boat undergoes uniform acceleration during the 3 seconds. What is the force on the boat? F = ma = 500 kg * 44.4 m/s 2 = N

3 20. A ball begins at rest and acquires a speed of 12 m/s when a force is applied for a distance of 0.5 m. If the ball has a mass of 1.0 kg, what is the force applied? vi = 0m/s vf = 12 m/s d = 0.5 m m = 1 kg. v f 2 = v i 2 + 2ad a = (v f 2 - v i 2 )/2d = (12m/s -0) 2 /2 * 0.5 m a = 144 m/s 2 F = ma = 1 kg * 144 m/s 2 = 144N 21. A bicyclist and his bicycle have a mass of 105 kg. What force is necessary to bring him to rest when he s traveling at 25 m/s and must stop to avoid a tree 4.0 m in front of him? m = 105 kg. vi = 25 m/s vf = 0 m/s d = 4m v f 2 = v i 2 + 2ad a = (v f 2 - v i 2 )/2d = (0m/s -25 m/s) 2 /2 * 4 m a = m/s 2 F = ma = 105 kg.* m/s 2 = N III. Fnet = ma Problems 22. For the system below, determine the size and direction of a force, F4, that must be present to cause the situation. F net = ma = F1+ F2 + F3 + F4 F net = 20 Kg * -3 m/s 2-60 N = -80 N+ 30 N+ 40 N F4-60 N = -10 N- F4 F4 = -50 N F 4 =? F 3 = 40 N F 2 = 30 N m = 20 kg F 1 = 80 N a = 3 m/s/s 23. A 60 kg young man rides in an elevator. a) Draw and label the forces acting on the man/elevator. b) The man s mass is 60 kg, how much does he weigh? c) The cable pulls with a force of 550 N. Determine the net force. d) Find the acceleration of the elevator.

4 24. A 50 kg woman steps in an elevator and pushes the down button. a) How much does she weigh? b) The tension in the cable is 350 N. Determine the net force on the rider/elevator. c) Find the acceleration of the elevator (watch your sign). 25. A 40 kg child rides in an elevator accelerating up at 2 m/s/s. a) Determine the net force on the child/elevator. b) How much does the child weigh? c) Find the tension the cable must exert to raise the child. 26. A man pulls a 30 kg box so that it accelerates at 1 m/s/s. a) What is the NET force on the box? b) If the man pulls with 50 N of force, how much friction force resists the motion? c) How much does the 30 kg box weigh? d) what is the amount of normal force on the box?

5 27. A 0.5 kg mass hangs from a string which is attached to a 1 kg mass. a) Find the weight of the 0.5 kg mass. b) What is the total mass of the system? c) Identify the force that makes the system accelerate. d) Determine the acceleration of the system. 28. A 2 N frictional force now acts on the block on the table. a) Determine the new net force. b) Find the new acceleration of the system. 29. Calvin implores Hobbes to go, Faster, Faster! Hobbes applies a pushing force of 300 N to the wagon with Calvin in it. Together, the wagon and Calvin are initially at rest and have a weight of 490 N. The force of friction of the wheels and the ground is 196 N. a) On a diagram, draw and label the forces acting on the wagon. b) Determine the acceleration of the wagon.

6 30. Calvin has been doing some growin lately and decides to take Hobbes (40 kg) for a ride in his 20 kg wagon. Calvin pulls with a force of 500 N at an angle of 30 degrees. The force of friction between the wheels and ground is 180 N. a) Draw and label the forces acting on the wagon. b) Calvin s pull is horizontal and vertical at the same time. Calculate the two components of Calvin s pulling force. c) Based upon the calculations in part b, calculate the normal force on the wagon (it will be reduced because of Calvin s pulling force). d) Calculate the net force, knowing that the horizontal component is helping the force and friction is hurting the force. e) Determine the acceleration of Hobbes as Calvin pulls him.