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Chapter 7: Q59PE (page 263)

(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.

Short Answer

Expert verified

(a) The duration for which you can play tennis is30.3min .

(b) Yes, it seems like a long time.

Step by step solution

01

Step 1: Power

Power is scalar quantity which is defined how fast the energy is being consumed by the system.

Mathematically,

P=Et鈥︹�︹�︹�︹�︹��.. (1.1)

Here, E is the amount of energy consumed, and t is the time.

02

The time which you can play tennis

(a)

The time can be calculated using equation (1.1).

Rearranging equation (1.1) in order to get an expression for time.

t=EP

Here, E is the energy of the tennis playerE=800kJ , and P is the power consumed while playing tennisP=440W .

Putting all known values,

t=800kJ440W=800kJ脳1000J1kJ440W=1818.18sec脳1min60sec=30.3min

Therefore, the duration for which you can play tennis is30.3min .

03

Exercise is necessary

(b)

Yes, this is surprisingly long. Exercise is required to burn more energy than is consumed.

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Most popular questions from this chapter

Using energy considerations, calculate the average force a 60.0-kg sprinter exerts backward on the track to accelerate from 2.00 to 8.00 m/s in a distance of 25.0 m, if he encounters a headwind that exerts an average force of 30.0 N against him.

The 70.0-kg swimmer in Figure 7.44 starts a race with an initial velocity of 1.25 m/s and exerts an average force of 80.0 N backward with his arms during each 1.80 m long stroke.

(a) What is his initial acceleration if water resistance is 45.0 N?

(b) What is the subsequent average resistance force from the water during the 5.00 s it takes him to reach his top velocity of 2.50 m/s?

(c) Discuss whether water resistance seems to increase linearly with velocity.

A person in good physical condition can put out 100 W of useful power for several hours at a stretch, perhaps by pedaling a mechanism that drives an electric generator. Neglecting any problems of generator efficiency and practical considerations such as resting time:

(a) How many people would it take to run a 4.00-kW electric clothes dryer?

(b) How many people would it take to replace a large electric power plant that generates 800 MW?

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Describe the energy transfers and transformations for a javelin, starting from the point at which an athlete picks up the javelin and ending when the javelin is stuck into the ground after being thrown.
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