• University of Phoenix 

  PHYS 

  PHYS 101 

  A 180 lb crate is on the ground, and a strong rope is attached. You need to move it across the basement floor, which has a coefficient of friction of 0.7 against the wood of the crate. To avoid actually working you call your friend to complain that you have to push it so hard. Your friend says you should pull it with the rope (your height will cause it to have a 60° angle to horizontal), since it will lower the weight and therefore the friction force. You counter (still avoiding work) that you will also have less force on the crate so it cancels out. What is the right answer? Is it better to pull with the rope or push?
• Two guys are sliding a refrigerator up a flight of carpeted steps. Each step is 20 cm high. The fridge weighs 200 lb. As it lays on the steps (the carpeted edges of the steps, in fact), it makes a 45° angle. One guy is at the top trying to pull by holding the sides of the fridge; the other unfortunate fellow is below, pushing upward rather hard. They are 3 steps up from the landing, and have just gotten the fridge moving at a constant speed, although quite slowly . The guy at the top is exerting 40 lb, which is all he can because of the awkward way he is holding it. (A) How much force is exerted by the guy pushing from below if we assume there is no friction? (B) How much force if the coefficient of friction is 0.1? The guy on the lower step stumbles, and stops pushing. The fridge starts to slide back down. (C) How fast is the fridge now descending after it has slid one step back? (with friction) After that one step down, the lower guy gets back into pushing. (D) How hard must he push to stop it before it descends the remaining 2 steps?
• A cat is pulling on the end of a long, lace table cloth that hangs close to the floor. The table is 1.1 m above the floor; on the table cloth is a 2.1 kg decorative bowl. Thanks to the cat's steady pull, the bowl is accelerating toward the edge of the table at 0.06 m/s2. The bowl is 0.5 m from the table's edge. (A) How fast is the bowl traveling at the moment it leaves the table? (B) How long till the bowl hits the ground once it leaves the table? (C) What is the force of friction between the bowl and the table cloth?
• You want to hang a 70 lb sign on a wire between two  buildings. Because of clearance below, and where you can insert the anchors,  you can only allow the wire to make a 10° angle with the horizontal. The  anchors that hold the sign into the wall are rated to hold 200 lb at maximum.  Will the sign fall? Explain.

5. Romeo is tossing pebbles gently up to Juliet's window. He wants the pebbles to hit the window with only a horizontal component of velocity. He is standing past some bushes 9 m below her window and 5 m from the base of the wall. How fast are the pebbles going when they hit her window?

6 A movie stunt requires a jump between the tops of two buildings that are 5 m apart. The far building is 10 m lower. If the stuntman can get to a speed of 5 m/s, should he make the jump?

7 A quarterback wants to throw a pass to his fastest receiver in the end zone. The receiver can run the 100 meter dash in 11 sec. The team is at the 30 yard line, and the quarterback will step back another 5 yards--and he is aiming at 5 yards deep in the endzone. So the ball needs to be thrown 40 yards, and the receiver runs 35.
(A) How long must the ball be in the air?
(B) How high does the ball get?
(C) What is the initial velocity?
(D) What is the angle of launch?

8. In the early days of space flight it was important to put a satellite into orbit with spin, but then to remove the spin once it got there. Without fancy propulsions systems, the following idea worked rather well: Two cables with attached masses were placed on either side of the satellite. The cables were wound around the satellite and the weights initially secured. On a signal, the weights would be released so as to unwind (as the satellite spins) until the ends of the cables unhook and each weight flies away into space (with its unhooked cable behind it). Why did it work and how?

9. A painting scaffold is suspended by a solid rope on one end, and a frayed rope at the other end. The scaffold is 5 m long, and is uniform throughout its length, with a mass of 50 kg. It is hung 12 m above ground.
(A) How much force is exerted by the frayed rope?
A painter drops a 30 kg canvas bag of painting supplies onto the center of the scaffold from the roof which is 8 m above the scaffold and 20 m above ground. The bag is slowed to a stop over a distance of 1 cm.
(B) What is the average force exerted by the scaffold to stop the supplies?

This just causes the frayed rope to snap, but you may now consider that the painting supplies have been stopped.
(C) How great is the torque about the good end?
(D) What is the moment of inertia (about the good end) of the scaffolding plus bag of supplies?
(E) With what angular acceleration does the scaffolding begin to rotate about its "good" end?

10. A 106-kg space station in the form of a huge wheel is rotating at 0.2 rad/s. For purposes of this problem, you may consider it's center of mass to be at rest, and that all the mass is concentrated in the rim, which is 100 m from the center. A large meteor (5 x 105 kg) is speeding towards it at 700 m/s, along a line that would go through the center. The meteor hits the rim of the station, where it sticks.

(A) What was the moment of inertia of the station before being hit?
(B) What was the angular momentum of the station before being hit?
(C) How fast is the center of mass of the station plus meteor travelling after collision?
(D) How fast is the station now rotating after collision?