chapter 7

How much work is done in lifting a 500-N barbell 2 m above the floor in 5 seconds?

1000 J

Compared with driving at 30 km/h, how much distance do you need to stop when you are driving at 60 km/h?

4 times as much distance

I drop an apple of 0.2 kg off a bridge that is 20 meters high. What is the kinetic energy of the apple just before it hits the water?

40 J

If an input of 100 J in a pulley system increases the potential energy of a load by 60 J, what is the efficiency of the system?

60%

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

Both take the same 1000 J.

Compared with a car moving at some original speed, how much work must the brakes of a car supply to stop a car that is moving twice as fast? How will the stopping distances compare?

It takes four times the work and four times the stopping distance.

A car is raised a certain distance in a service-station lift, thus giving it potential energy relative to the floor. If it were raised twice as high, how much more potential energy would it have?

It would have twice as much potential 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? How about for the case in which the lighter sack is moved the same distance in half the time?

Lifted in the same time, the power is the same. The light sack moving in half the time requires double the power.

Two cars are raised to the same elevation on service-station lifts. If one car is twice as massive as the other, compare their gains of potential energy.

The car with twice the mass has twice the potential energy.

An apple hanging from a limb has potential energy because of its height. If it falls, what becomes of this energy just before it hits the ground? When it hits the ground?

The energy is kinetic energy before it hits the ground; it is thermal energy after.

A model airplane moves 3 times as fast as another identical model airplane. Compared to the momentum and kinetic energy of the slower airplane, the momentum and kinetic energy of the faster plane is

The momentum is 3 times as much and the kinetic energy is 9 times as much

When traveling twice as fast your kinetic energy is increased _______.

by four.

Energy cannot be _______.

destroyed.

The work that is done when twice the load is lifted twice the distance is _______.

four times as much.

When you lift twice the load twice as high, in half the time, the increase in potential energy is _______.

four times.

When one does twice the work in twice the time, the power expended is _______.

the same.