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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……………….. (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×1000J1kJ440W=1818.18sec×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

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?

Do devices with efficiencies of less than one violate the law of conservation of energy? Explain

Boxing gloves are padded to lessen the force of a blow.

(a) Calculate the force exerted by a boxing glove on an opponent’s face, if the glove and face compress 7.50 cm during a blow in which the 7.00-kg arm and glove are brought to rest from an initial speed of 10.0 m/s.

(b) Calculate the force exerted by an identical blow in the gory old days when no gloves were used and the knuckles and face would compress only 2.00 cm.

(c) Discuss the magnitude of the force with glove on. Does it seem high enough to cause damage even though it is lower than the force with no glove?

(a) What is the average useful power output of a person who does \begin{aligned}6.00 \times {10^6}{\rm{ J}} \end{aligned}of useful work in 8.00 h?

(b) Working at this rate, how long will it take this person to lift 2000 kgof bricks 1.50 mto a platform? (Work done to lift his body can be omitted because it is not considered useful output here.)

(a) Calculate the power per square meter reaching Earth’s upper atmosphere from the Sun. (Take the power output of the Sun to be\(4.00 \times {10^{26}}{\rm{ W}}\).)

(b) Part of this is absorbed and reflected by the atmosphere, so that a maximum of\(1.30{\rm{ kW}}/{{\rm{m}}^2}\)reaches Earth’s surface. Calculate the area in\({\rm{k}}{{\rm{m}}^2}\)of solar energy collectors needed to replace an electric power plant that generates\(750{\rm{ MW}}\)if the collectors convert an average of\(2.00\% \)of the maximum power into electricity. (This small conversion efficiency is due to the devices themselves, and the fact that the sun is directly overhead only briefly.) With the same assumptions, what area would be needed to meet the United States’ energy needs\(\left( {1.05 \times {{10}^{20}}{\rm{ J}}} \right)\)? Australia’s energy needs\(\left( {5.4 \times {{10}^{18}}{\rm{ J}}} \right)\)? China’s energy needs\(\left( {6.3 \times {{10}^{19}}{\rm{ J}}} \right)\)? (These energy consumption values are from 2006.)
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