Work done on a system puts energy into it. Work done by a system removes energy from it. Give an example for each statement.

Short Answer

Expert verified

When a person pushes the lawn mower it increases the energy system and the mower starts moving with constant velocity. Once the lawn mower starts moving due to friction, it starts losing energy in the form of heat, and finally its stops.

Step by step solution

01

Definition of Concept

Work energy theorem:According to the work-energy theorem, the change in kinetic energy of the body equals the work done on the body.

In other words, when the kinetic energy of the body increases, the work is done on the system and when the kinetic energy of the system decreases, the work is done by the system.

02

Work on a system that adds energy to it.

When the work is done on the system, the final kinetic energy of the system will be greater than the initial kinetic energy of the system before the application of force on the system.

Assuming a lawn mower is initially at rest. A gardener pushes the lawn mower (work is done on the mower), so the gardener increases the energy of the mower and it starts to move.

Therefore, Work performed on a system adds energy to it.

03

A system's work depletes its energy.

When the work is done by the system, the final kinetic energy of the system will be smaller than the initial kinetic energy of the system before the application of force on the system.

Assuming a lawn mower is moving. There is a frictional force acting between the lawn mower and the lawn. To overcome this friction the mower has to do work against it (work is done by the system), so the friction decreases the energy of the mower and finally it stops.

Therefore, the work done by a system removes energy from it.

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Most popular questions from this chapter

The awe-inspiring Great Pyramid of Cheops was built more than 4500 years ago. Its square base, originally 230 m on a side, covered 13.1 acres, and it was 146 m high, with a mass of about 7×109 kg. (The pyramid’s dimensions are slightly different today due to quarrying and some sagging.) Historians estimate that 20,000 workers spent 20 years to construct it, working 12-hour days, 330 days per year.

(a) Calculate the gravitational potential energy stored in the pyramid, given its center of mass is at one-fourth its height.

(b) Only a fraction of the workers lifted blocks; most were involved in support services such as building ramps (see Figure 7.45), bringing food and water, and hauling blocks to the site. Calculate the efficiency of the workers who did the lifting, assuming there were 1000 of them and they consumed food energy at the rate of 300 kcal/h. What does your answer imply about how much of their work went into block-lifting, versus how much work went into friction and lifting and lowering their own bodies?

(c) Calculate the mass of food that had to be supplied each day, assuming that the average worker required 3600 kcal per day and that their diet was 5% protein, 60% carbohydrate, and 35% fat. (These proportions neglect the mass of bulk and nondigestible materials consumed.)

Figure 7.45 Ancient pyramids were probably constructed using ramps as simple machines. (credit: Franck Monnier, Wikimedia Commons)

List the energy conversions that occur when riding a bicycle.

In Example 7.7, we found that the speed of a roller coaster that had descended 20.0m was only slightly greater when it had an initial speed of 5.00m/s than when it started from rest. This implies that ΔPEΔKEi.Confirm this statement by taking the ratio of ΔPEto ΔKEi. (Note that mass cancels.)

How much work does a supermarket checkout attendant do on a can of soup he pushes \(0.600{\rm{ m}}\) horizontally with a force of \(5.00{\rm{ N}}\)? Express your answer in joules and kilocalories.

(a) How long can you play tennis on the 800 kJ (about 200 kcal) of energy in a candy bar?

(b) Does this seem like a long time? Discuss why exercise is necessary but may not be sufficient to cause a person to lose weight.

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