Chapter 6 end of chapter review questions

What is the efficiency of the machine that miraculously converts all the input energy to useful output energy?

100% - The efficiency of an ideal machine

Cite an example where a force is exerted on an object without doing work on an object.

A force is needed to hold a barbell overhead, but this force does no work on the barbell while the bar bell is at rest.

Compared to some original speed, how much work must the brakes of a car supply to stop a four times as fast car? How will the stopping distance compare?

A four times as fast car has 16 times as much DE and will require 16 times as much work to stop, and 16 times as much stopping distance.

A moving car has kinetic energy. If it speeds up until it is going four times as fast, how much kinetic energy does it have in comparison?

A three times as fast car has 4(2) or 16 times the KE.

Which requires more work- lifting a 50-kg sack a vertical distance of 2 m or lifting a 25-kg sack a vertical distance or 4 m?

Both are the same amount of work because the force x distance is the same for each.

If a machine multiplies force by a factor of four, what other quantity is diminished, and how much?

Increasing force means decreasing distance, so the distance moved by four fold force is one fourth as much

Is a machine physically possible that has an efficiency greater than 100 percent? Discuss.

Not in the world as we know it. Such a machine would clearly violate the conservation of energy, a law that has never been violated.

What is the ultimate source of geothermal energy?

Nuclear energy from radioactive decay.

What will be the kinetic energy of pile driver ram when it undergoes a 10 kg decrease in potential energy?

PE = KE= 10kj

How many joules of potential energy does a 1-N book gain when it is elevated 8 m?

PE= 1 N x 4 m = 40 J. At 8 m the PE = 80 J.

What are the two main forms of mechanical energy?

Potential and Kinetic Energy

If both sacks in the preceding question are lifted their respective distances in the same time, how does the power required for each compare?

Power for each is the same because the same work is done in the same time. Twice the power is required to do the same work on the lighter sack in half the time.

If a man pulls 1 m of rope downward with a force of 100N, and the load rises 1/7 as high, what is the maximum load that can be lifted?

Seven times as much, 700 N

What is the ultimate source of energies for the burning of fossils fuels, dams, and windmills?

Solar energy.

Two cars are lifted to the same elevation in a service station. If one car is twice as massive as the other, how do their potential energies compare?

The twice as massive car has twice the PE.

A car is lifted a certain distance in a service station and therefore has potential energy relative to the floor. If it were lifted twice as high, how much potential energy would it have?

Twice

What happens to a person whose work output is less than the energy he or she or she consumes? Whose work output is greater than the energy he or she consumes? Can an undernourished person perform extra work without extra food?

When work output is less than input energy, the person gains weight. When work output is more than input energy, weight loss results. an undernourished person performing extra work without extra food violates energy conservation-and perishes.

What do we call the quantity "Force x Distance"

Work

What is it that a body having energy is capable of doing?

Work.

Can a machine multiply input force? Input distance? Input energy?

Yes; yes; No!