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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鈥檚 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鈥檚 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鈥檚 surface.

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Most popular questions from this chapter

Energy that is not utilized for work or heat transfer is converted to the chemical energy of body fat containing about \(39{\rm{ kJ}}/{\rm{g}}\). How many grams of fat will you gain if you eat \(10,000{\rm{ kJ}}\) (about \(2500{\rm{ kcal}}\)) one day and do nothing but sit relaxed for \(16.0{\rm{ hr}}\) and sleep for the other \(8.00{\rm{ h}}\)? Use data from Table 7.5 for the energy consumption rates of these activities.

List four different forms or types of energy. Give one example of a conversion from each of these forms to another form.

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 \(28.0{\rm{ m}}/{\rm{s}}\)?

Explain why it is easier to climb a mountain on a zigzag path rather than one straight up the side. Is your increase in gravitational potential energy the same in both cases? Is your energy consumption the same in both?

Suppose a star 1000 times brighter than our Sun (that is, emitting 1000 times the power) suddenly goes supernova. Using data from Table 7.3:

(a) By what factor does its power output increase?

(b) How many times brighter than our entire Milky Way galaxy is the supernova?

(c) Based on your answers, discuss whether it should be possible to observe supernovas in distant galaxies. Note that there are on the order of 1011 observable galaxies, the average brightness of which is somewhat less than our own galaxy.
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