Work

 

 

 

 

 

 

 

 

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Work

Chapter Review

One More Time

Work and Friction

  1. A horizontal force of 420 N is used to push a 100-kg crate for 5 m across a level warehouse floor. How much work is done?

  2. The 420 N force of Prob. 1 is instead exerted on the crate at an angle of 35 degrees above the horizontal. How much work is done now?

  3. A 60 kg box is pushed 12 m across a horizontal floor by a horizontal force of 200 N. The coefficient of kinetic friction is 0.3. (a) How much work went into overcoming friction and (b) how much into accelerating the box?

  4. How much work is done in raising a 2 kg book from the ground to a height of 1.8 m?

  5. How much work is done raising a 2000 kg elevator through a height of 26 m?

  6. In raising a 200 kg bronze statue 10,000 J of work is performed. How high is it raised?

  7. The Identical 65 kg twins Alpha and Beta climb a mountain 3800 m high. Alpha follows a trail that averages 20 degrees above the horizontal, while Beta follows a different trail that averages 30 degrees above the horizontal. How much work does each twin do?

Power and Efficiency

  1. A 40 kg woman runs up a staircase 4 m high in 5 s. Find her minimum power output.

  2. A hoist Powered by a 10 kW motor is used to raise a bucket filled with concrete and having a total mass of 500 kg to a height of 80 m. If the efficiency of the hoist is 80 percent, find the time needed.

  3. A horse has a power output of 1 kW when it pulls a wagon with a force of 400 N. What is the wagon's velocity?

  4. A motorboat requires 60 kW to move at the constant velocity of 4.5 m/s. How much resistive force does the water exert on the boat at this velocity?

Kinetic Energy

  1. Find KE final of a 1000 kg car whose velocity is 20 m/s.

  2. What velocity does a 1-kg object have when its kinetic energy is 1 J?

  3. A 58.18 kg woman skates at a velocity of 15 m/s What is her kinetic energy?

  4. A hammer with  a 1.5 kg head is used to drive a nail into a wooden board. If the hammerhead is moving at 3 m/s when it strikes the nail and the nail moves 10 mm into the board, find the average force the hammerhead exerts on the nail.

  5. Find the power output of the engine of a 1200 kg car while the car accelerates from 30 to 100 km/h in 10 s.

Potential Energy

  1. A 1.5 kg book is held 60 cm above a desk whose top is 70 cm above the floor. Find the potential energy of the book (a) with respect to the desk, and (b) with respect to the floor.

  2. Compare the potential energy of a 1200 kg car at the top of a hill 30 m high with its kinetic energy when its velocity is 100 km/h (27.8 m/s).

  3. (a) A 50.68 kg woman jumps off a wall 1 m high and lands on a concrete road with her knees stiff. Her body is compressed by 7.5 cm. at the moment she hits the road. What is the average force exerted on her by the road? (b) If the woman bends her knees on impact so that she comes to a stop over a distance of  30 cm., what would her force be?

Conservation of Energy

  1. At her highest point, a girl on a swing is 2.3 m above the ground, and at her lowest point she is 0.35 m above the ground. What is her maximum velocity?

  2. A man skies down a slope 200 m high. If his velocity at the bottom of the slope is 20 m/s, what percentage of his initial potential energy was "lost" due to friction and air resistance.

  3. A 30 kg crate is pulled up a ramp 15 m long and 2 m high by a constant force of 100N. The crate starts from rest and has a velocity of 2 m/s when it reaches the top of the ramp. What is the frictional force between the crate and the ramp?

  4. An advertisement claims that a certain 1200 kg car can accelerate from rest to a speed of 25 m/s in a time of 8.0 s. What average power must the motor produce to cause this acceleration? Ignore friction losses.

  5. Repeat Problem 4. immediately above if the data apply to a car going up a 20 degree incline.

That's all folks