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Chapter 7: Q4 CQ (page 258)

The person in the figure does work on the lawn mower. Under what conditions would the mower gain energy? Under what conditions would it

lose energy?

Short Answer

Expert verified

When the lawn mower is pushed it gains energy, whereas energy is lost when work is done by the mower to overcome the friction between tyres and the earth’s surface.

Step by step solution

01

Understanding friction.

The dissipative force acting between the two bodies in contact, which are in relative motion with respect to each other, is known as friction. It always acts at the plane of contact between the two bodies, in the direction opposite to the relative motion of the surface.

02

Evaluating.

When the person pushes the lawn mower, he is doing work on the mower. This work gets stored in the mower in the form of mechanical energy. Thus, the mower is gaining energy. On the other hand, the mower has to do work against the frictional force acting between the wheels of the mower and the earth’s surface. Hence, the mower will lose energy in overcoming the friction.

03

Conclusion.

When the lawn mower is pushed, it gains energy and it loses energy in overcoming the friction between tyres and earth’s surface.

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Most popular questions from this chapter

Suppose a car travels\(108{\rm{ km}}\)at a speed of\(30.0\,{\rm{m}}/{\rm{s}}\), and uses\(2.0{\rm{ gal}}\)of gasoline. Only\(30\% \)30% of the gasoline goes into useful work by the force that keeps the car moving at constant speed despite friction. (See Table 7.1 for the energy content of gasoline.)

(a) What is the magnitude of the force exerted to keep the car moving at constant speed?

(b) If the required force is directly proportional to speed, how many gallons will be used to drive\(108{\rm{ km}}\) at a speed of \(28.0{\rm{ m}}/{\rm{s}}\)?

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?

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.

Suppose the ski patrol lowers a rescue sled and victim, having a total mass of 90.0 kg, down a 60.0º slope at constant speed, as shown in Figure 7.37. The coefficient of friction between the sled and the snow is 0.100.

(a) How much work is done by friction as the sled moves 30.0 m along the hill?

(b) How much work is done by the rope on the sled in this distance?

(c) What is the work done by the gravitational force on the sled?

(d) What is the total work done?

Figure 7.37 A rescue sled and victim are lowered down a steep slope.

Using values from Table 7.1, how many DNA molecules could be broken by the energy carried by a single electron in the beam of an old-fashioned TV tube? (These electrons were not dangerous in themselves, but they did create dangerous x rays. Later model tube TVs had shielding that absorbed x rays before they escaped and exposed viewers.)
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