Forces 2 Free Body Diagrams All objects are represented as a point 1. A 1.5 kg box hangs by a string. What is the net force on the box?

Transcription

1 Physics R Forces 2 Free Body Diagrams All objects are represented as a point 1. A 1.5 kg box hangs by a string. What is the net force on the box? Date: What is the force of gravity? What is the tension force in the string? Draw a free body diagram to the right 2. A traffic light is held up by 2 cables What is the net force on the light? Draw a free body diagram to the right Vertical Motion 3. A rocket with mass 1,000 kg is accelerating upwards at 4.0 m/s 2. b. Calculate the net force acting on the rocket c. Calculate the weight of the rocket d. Is there a normal force acting on the rocket? e. Calculate the force the engines must apply to give this acceleration. 1

2 4. A rocket that weighs 7,840 N is fired up (starting from rest). The force of propulsion is 10,440 N. b. Determine the mass of the rocket. c. Determine the acceleration of the rocket. d. Determine the velocity of the rocket after 8 seconds 5. A 2 kilogram pail in a well is pulled up with a rope. The pulling force on the rope is 30 N. b. Calculate the weight of the pail c. Determine the acceleration of the pail d. How much time will it take the pail to be raised 15 m? 6. A 70 kg man is standing on an elevator which is moving upwards at constant velocity of 3 m/s. b. Determine the net force acting on the man c. Determine the weight of the man. d. Determine the normal force acting on the man. 2

3 7. The same 70 kg man is in an elevator that is moving upwards at 3 m/s and slowing to a stop at a rate of -1.5 m/s 2. b. Determine the net force acting on the man c. Determine the weight of the man. d. Determine the normal force acting on the man. e. How much time does it take the elevator to stop? 8. A 10 kg puppy is on an elevator which accelerates upwards at 2.2 m/s 2, starting from rest. b. Determine the net force on the puppy c. Determine the weight of the puppy d. Determine the normal force acting on the puppy. e. How far will the puppy travel in 3 seconds? 9. The mass of an elevator and its occupants are 752 kg. The elevator is pulled upwards by a cable which experiences a tension force of 8950 N. for the problem. b. Determine the acceleration of the elevator 3

4 Forces at an Angle To calculate horizontal and vertical components of force, use the equations: 10. A little boy pulls a wagon with a force of 10 N at an angle of 30 degrees. What is the horizontal component of the applied force? Vertical component? 11. You walk an unwilling 10 kg dog by pulling on its leash with a force of 20 N at an angle of 50 degrees from the horizontal. to the right. b. What is the horizontal component of the pulling force? c. What is vertical component of the pulling force? Which force accelerates the dog, vertical or horizontal? d. Calculate the dog s acceleration. 12. Molly the 5 kg dog doesn t want to go on a walk. You pull on the leash with a force of 15 N at an angle of 20 degrees from the horizontal. How much force of friction does Molly have to apply so that you can t move her? (Hint: Think of the last problem) 13. You push a 7 kilogram desk with a 30 N force pointing downward at 15 degrees. What is the horizontal component of your pushing force? What is the acceleration of the desk? (Assume no friction) 14. A 12 kg lawnmower is pushed with a force of 40 N at an angle of 15 degrees below the horizontal. a. What is the horizontal component of the push? b. What is the acceleration of the lawnmower (neglect friction)? 4

5 5

6 To get the force down an incline, use sin. When going down an incline, think of a road sign (use sin) 14. A 10 N block is accelerating down a frictionless ramp inclined at 20 degrees. Calculate the magnitude of the net force causing this acceleration 15. A 0.5 kg marble is rolling down a 12 degree frictionless incline. Calculate the magnitude of the net force causing it to accelerate. 16. A 600 N sled and sledder slide down an 18 degree hill with constant velocity. Calculate the magnitude of the force of friction acting on the sled 6

7 Review Checklist Calculate force of static friction. Determine amount of static friction acting on a motionless object. Determine minimum coefficient of friction in a system. Determine net force, force of kinetic friction and/or acceleration in a sliding system. 1. A wood block on a wooden surface is pushed with 20N of force and moves at a constant 12 m/s. What is the force of friction acting on the block? 2. A 12 kg lawnmower requires 30 N of force to push it with constant velocity. What is the force of friction between the lawnmower and the lawn? 3. A 2.0 kilogram wooden object rests a on a wooden surface. What is the maximum force that you can push on the object, before it starts to move? 4. A motionless 30 newton steel object on a steel surface is pushed with a 2.0 newton force. What is the force of static friction? 5. What is the minimum coefficient of static friction between a 25 newton object and a surface if it is pushed with a force of 10 newtons and does not move? 6. A 20 newton object is pulled along a flat surface at a constant speed with a force of 4.0 newtons. o What is the force of kinetic friction? o What is the kinetic coefficient of friction? Determine the components of an object s weight when placed on an incline. Determine the effects of changing the angle of the incline on the components of weight. Determine behavior of objects on frictionless inclines and inclines with friction. Determine friction, net force and/or applied forces on inclines based on the situation. 7. A 3.0 kilogram object is placed on a frictionless, 25 incline. Determine the component of the object s weight that is parallel to the incline. 8. A 5.0 kilogram object is moving down a 20 degree incline with constant velocity. What is the force of friction acting on the object? Be sure to go over the review checklist from the first Forces note packet!!! Answers: N N N N N, N N 7