A) How Much Energy Did the Push Give the Puck?

Physics 111 HW09

assigned2 March 2011

S-01. A mass is attached to a spring on an incline as shown in the diagram at right. Calculate the distance the spring stretches past its equilibrium point. The incline is frictionless.

S--02. A mass is swung around in a circle on a frictionless horizontal table. The speed of the mass remains constant, and the spring remains the same length. Does the spring do any work on the mass during this motion? Explain.

S-03. A block with mass M rests on a frictionless surface and rests against a horizontal spring with force constant k. The other end of the spring is attached to a wall (see figure). A second block with mass m rests on top of the first block. The maximum coefficient of static friction between these two blocks is μs max. Find the maximum distance you can compress the spring by pushing block M against it such that when you release M, m will not slip off.

IW-01. A 0.25 kg air puck initially at rest is given a 0.100 N push over 1 meter over a frictionless surface.

a) How much energy did the push give the puck?

b) Where did this energy go?

c) Calculate the new speed of the puck using both kinematics and energy considerations.

IW-02. A factory worker pushes a 30.0 kg crate a distance of 4.5 m along a level floor at a constant velocity by pushing horizontally on it. The coefficient of kinetic friction between the crate and floor is 0.25.

a) What magnitude of force must the worker apply?

b) How much energy did the worker give to the crate? (What is the work that the worker does on the crate?)

c) How much work do the normal force of the floor on the crate and gravity do?

d) How much work does friction do on the crate?

e) Where does the energy the worker put into the crate go?

IW-03. Repeat number 4 above if the worker pushes down on the crate at an angle of 30o below the horizontal.

W-01. A 4.80 kg watermelon is dropped (zero initial speed) from the roof of a 25.0 m tall building. Ignore air resistance.

a) Calculate the work gravity does on the watermelon for the trip to the ground.

b) Use energy considerations to calculate the speed of the watermelon just before it hits the ground.

W-02. A car is traveling on a level road with speed vo at the instant when the brakes lock, so that the tires slide rather than roll.

a) Use energy considerations to derive an expression for the stopping distance of the car in terms of vo, g, and the coefficient of kinetic friction between the tires and the road μk.

b) If vo is doubled, what happens to the stopping distance?