Energy Problems

Purpose

To analyze and solve problems involving work, power, and all forms of energy.

On all of these problems, show your work (the numbers that you are using and how you are using them) and the answer with the proper significant digits and the proper units where appropriate.

Part 1: Work and Power

1. Determine whether work is being done on the object in each statement below. If you think work is being done, write W in the space. If you think no work is being done, write N in the space.

        _______ You throw a football 28 meters.

        _______ You push against a cement wall.

        _______ You carry a bag of groceries to your car.

        _______ You push a stalled car to the side of the road.

        _______ You study for a physics test.

2. Calculate the amount of work done by the applied force shown for each diagram below.

a.                                b.

1. A 20 kg mass is lifted 12 m up at a constant speed.
2. An 11 kg mass is moved 5 m horizontally with a 74 N force that is 25 degrees off horizontal.

3. At the 2013 World Weightlifting Championships, Tatiana Kashirina set the world record for the Women’s Clean and Jerk using an average force of 1900N to lift the weight 2.2 meters, how much work was done?

4. To slide a book across a table, you exert a force of 4.3 N in the direction of motion. How far have you moved the book if you do 1.6 J of work?

5. How long does it take to exert 1500 J of work if power output is 350 W?

6. A microwave emits 22,000 J of energy in 2 minutes. What is the power of the microwave?

7. An object that weighs 35 N on Earth moves downward a distance of 120 m in 22 seconds. What is the power output?

8. An elevator must lift 1050 kg a distance of 80 m at a velocity of 3.5 m/s. What is the average power the elevator exerts during this trip? Include a free body diagram.

Part 2: Kinetic, Potential, and Mechanical Energy

9. What is the kinetic energy of a 920 kg roller coaster car moving with a velocity of 15.7 m/s?

10. A car with a mass of 850 kg has a kinetic energy of 123,000 J. Calculate the velocity of the car.

 11. If you are standing on a ladder, 3.25 m from the ground, and you weigh of 650 N, what is your potential energy?

12. A rock on a cliff has 4,000 J of potential energy relative to the cliff’s base. If the cliff is 50 m above the ground, what is the mass of the rock?

13. The rock in problem 12 slides off the cliff, but lands on a ledge below. It now has a potential energy of 500 J. What is the height of the ledge it now rests on?

14. A frictionless cart on a track is pushed and follows the path as shown in the diagram below. Complete the table under the diagram by applying the conservation of energy. Assume no energy is lost to forces outside of the system (friction, air resistance, etc).

         The mass of the cart is 5.0 kg.

Find the height of the cart at position C.

Part 3: Thermal Energy

Table 1: Specific Heat of Common Substances

Physics Principles and Problems. Columbus OH: Glencoe/McGraw-Hill., 2009. 318+. Print.

Table 2: Heats of Fusion and Vaporization of Common Substances

Physics Principles and Problems. Columbus OH: Glencoe/McGraw-Hill., 2009. 318+. Print.

 15. How much heat is needed to raise the temperature of 50.0 g of water from 277 K to 315 K?

16. If it takes 1,650 J of energy to raise the temperature of copper from 250 K to 285 K, what is the mass of the copper?

 17. A 500 g block of metal absorbs 8,370 J of heat when its temperature changes from 295 K to 338 K. Calculate the specific heat of the metal.

 18. Use the Specific Heat of Common Substances table to identify the metal in problem 17.

 19. How much heat energy is needed to melt 0.300 kg of copper?

20. How much iron can be vaporized if 1.30 x 108 J of energy is applied?

21. A calorimeter containing 0.50 kg of water at 15ºC has a 0.05 kg block of aluminum that is 115ºC added to it. What is the final temperature of the system?

Challenge Problem:

22. How much heat is added to 0.100 kg of ice at 253 K to convert it to steam at 393 K?