Physics CPA Unit 4
Conceptual Questions:
1. Explain the concept of inertia using the importance of seat belts and air bags in a car.
2. Can an object have 2,3 or 4 or more individual forces acting upon it and still be at rest or moving at constant velocity? Explain/sketch.
3. Can an object have 1 individual force acting upon it and still be at rest or moving at constant velocity? Explain/sketch.
4. What is the net force on a body moving in a straight, level path at constant speed? How do you know?
5. Explain why it is harder to turn and push the heavily-loaded shopping cart through the aisles compared to the empty shopping cart. Would this still be true on the moon?
6. Which is a more fundamental property of a body, mass or weight? Explain…
Problems:
7. Two crazy shoppers fight over a popular toy, of mass 0.50 kg. One pulls West with a force of 30 N, the other suddenly pulls East with a force of 38 N and the toy flies away. What is the initial acceleration of the toy?
8. A 2.0 –kg rock falls against an instantaneous air resistance force of 11 N.
a) Calculate the acceleration of the rock at this point in time.
b) When the rock reaches terminal velocity, it falls at constant speed. Determine the air resistance force at terminal velocity.
9. A 1.5 –kg rock falls from a height of 4.0 m, and its impact time is measured to be 1.0 s. Calculate the effective force of air resistance that acted upon the rock.
10. A student is standing on a scale in an elevator, which is at rest. The scale measures the student’s weight to be 836N. The elevator accelerates, and the scale now reads 936 N.
a) What is the elevator car’s acceleration?
b) If the elevator car has a mass of 2000 kg (add student), what is the tension in the elevator cable during the acceleration?
11. Two sleds are attached to each other with ropes. The first in line contains a child of (mass + sled ) 40 kg, the second contains a child of (mass + sled) 30 kg.
a) If you pull on the rope with a horizontal force of 100N, and move at a constant speed, what is the tension in the rope near you and between the sleds?
b) What Applied Force is required to accelerate the system at 2.0 m/s2? Find the tension between the sleds for this acceleration.
12. A truck of mass 5450 is towing two trailers. Trailer 1 has mass = 31,500 kg and trailer 2 has mass = 19,600 kg. The truck accelerates from zero to 4.5 m/s in 15 s.
a) What Force is required to be supplied by the truck (against the road), to cause this acceleration? (Ignore opposing friction)
b) What is the tension in the tow bar between the truck and trailer one, and between trailer one and trailer two?
13. An Atwood’s machine has two masses that are attached to a light cord that passes over a frictionless pulley. M1 = 3M2. What is the acceleration of the system?
14. A 70 N box rests on a table. A rope tied to the box runs vertically upward over a (frictionless) pulley and a weight is hung from the other side. Determine the Normal force that the table exerts on the 70-N weight if the other mass is
a) 70 N and b) 30 N
15. A heavy sign of mass 15 kg is supported by two ropes that make angles of 30 degrees with the vertical. Calculate the tension in the ropes.
16. A 2.0 kg flower pot is suspended in a holder that has three ropes. Each rope makes an angle of 65 degrees with the horizontal. What is the tension in each rope?
17. A student supports a 1.0 kg weight on a small rope by holding it so that the ropes make an angle of 45 degrees with the horizontal.
a) With what force must the student pull with each arm along the rope?
b) If the students attempts to decrease the angle to 2 degrees, what force is required?
18. A 77 kg sign is hung with two wires. Wire A makes an angle of 30 degrees with the vertical, and Wire B is a horizontal wire.
What are the tensions in Wires A and B?
19. A street lamp weighing 150.0-N is supported by two wires that each make an angle of 35 degrees with the vertical.
a) Calculate the tension in the wires.
b) Suppose the configuration is changed so that one of the angle of one of the wires (Wire 1)is increased to 50 degrees with respect to the vertical. Determine the tension in each wire.
20. A 4.0 kg bundle of food is suspended from a rope, as shown, at a campsite to protect it from bears.
a) What is the tension in the rope if the angle theta shown is 8 degrees with the horizontal?
b) Suppose the bear does get the bundle and pulls down with a force of 90 N. The rope stretches so that the angle theta is 25 degrees. What is the new tension in the rope?
21. A worker has to move a 17.0-kg crate along a floor. The coefficient of kinetic friction between crate and floor is 0.214. If the worker pulls horizontally on the crate with a force of 49-N, what is the resulting motion of the crate?
22. A 2.0 kg silverware drawer is stuck and will not slide open. The owner pulls with more and more force, until at 8.0 N, the drawer opens, throwing all the utensils to the floor. Calculate the us between the drawer and the cabinet. How can this problem be prevented?
23. A driver is driving a 2500 kg car at 14.0 m/s on a level road. He hits the breaks and skid on an icy patch, and the car slides to a stop in a distance of 25.0 m.
What is the uk between the tires and the icy road?
a) Calculate acceleration
b) Free-body diagram
c) Newton’s second law, x,y-axis.
d) friction formula
24. A child rests on a sled, total mass = 50.0 kg.
a) Ignoring friction, what force is needed to accelerate the child+sled at 1.15 m.s2 ?
b) Suppose instead that the coefficient of kinetic friction between sled and snow is 0.16. What force is needed to accelerate the child +sled at 1.15 m/s2?
c) Keeping the same µ, what force would be required to pull the child+sled at constant velocity
25. A sled with total mass 40.0 kg is sitting on flat, snow-covered ground.
The us between the sled and snow is 0.30, the uk is 0.10
a) What force is required to just get the sled moving?
b) Once moving, what force is needed to pull the sled at constant velocity?
c) What force would be needed to accelerate the sled briefly at 3.0 m/s2?
d) The sled is moving and it hits a grassy area, and it slows to stop with a = -4.0 m/s2. What is the coefficient of friction between the sled and grass? Is this static or kinetic friction?
26. A sled an rider with combined mass of 40.0 kg is at rest upon level snow with
a us of 0.3.
a) What force is required to just start the sled moving?
b) If the uk is 0.1, what force is required to keep the sled moving at a constant speed?
c) What angle hill would also cause the sledder to slide at constant velocity
with uk = .1?
Textbook
Concepts:
pg. 112 # 41, 43, 44, 46, 58
Problems:
pg. 97 # 17, pg. 100 # 20, pg. 101 # 25, pg. 106 # 32
Pg. 113-116 # 62, 69-71, 75-78, 82, 85, 88, 90, 92, 96
Pg. 117 # 6,7