Physics 213 Quiz # 4 6 points each Wednesday 10-11-2006

1. Use the figure below for problems a thru d.

a. If the pulling force P is 150 N and the crate doesn't move, what is the magnitude of friction force f?

b. If the pulling force P is 200 N and the crate doesn't move, what is the magnitude of friction force f?

c. If the force of sliding friction is 250 N, what force P is necessary to keep the crate sliding at constant velocity?

d. If the force of sliding friciton is 250 N and the force P is 300 N, what is the acceleration of the block?

2. A girl stands on a bathroom scale inside an elevator. When the elevator is at rest, the scale reads

750 N (170 lb). State whether the scales reading increases, decreases, or remains the same when:

a. The elevator is moving upward at an increasing speed.

b. The elevator is moving upward at a decreasing speed.

c. The elevator is moving upward at constant speed.

d. The elevator is moving downward at decreasing speed.

e. The elevator is moving downward at constant speed.

f. On a freebody diagram, the name we give the scales reading is the ________________ force because the direction of this force is always perpendicular to the plane of contact.

3. A farmer is trying to pull a pair of pants in two so he can use them for rags. First he attempts doing this with a horse as indicated in the picture below.

Failing to tear the pants in two, he decides to try two horses, as indicated in the picture below.

Do his chances of pulling the pants apart (circle correct answer) increase, decrease, or remain the same?

4. The mechanic is raising this 300-pound engine at a constant rate using the frictionless pulley system shown. With how much force is he pulling on the rope?

1 . 600 lbs

2. 450 lbs.

3. 300 lbs.

4. 200 lbs.

5. 150 lbs.

6. 100 lbs.

7. 50 lbs.

8 . impossible to determine

5.

6.

7. Two forces act on a 5 kg object as indicated in the figure at right.

F₁ = 30 N and F₂ = 15 N. Find the magnitude and direction of the acceleration of the object.

8.