Homework 7

A melon is tossed straight upward with 100 J of kinetic energy. If air resistance is negligible the melon will return to its initial level with a kinetic energy of

100 J

The power expended doing 100 J of work in 50 s is

2 W

If you push a crate horizontally with 100 N across a 10-m factory floor and the friction between the crate and the floor is a steady 70 N, how much kinetic energy does the crate gain?

300 J

Two identical particles move toward each other, one twice as fast as the other. Just before they collide, one has a kinetic energy of 25 J and the other 50 J. At this instant their total kinetic energy is

75 J

A motorcycle moving at 50 km/h skids 10 m with locked brakes. How far will it skid with locked brakes when traveling at 150 km/h?

90 m

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

Both take the same 1000J

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?

Immediately before hitting the ground the apple's energy is kinetic energy; when it hits the ground, its energy becomes thermal energy

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

If you toss a stick into the air, it appears to wobble all over the place. Specifically, about what place does it wobble?

It will rotate about the center of mass.

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

Which energy production method does not ultimately depend on the sun?

Nuclear fission

What is the ultimate source of geothermal energy?

Nuclear power in Earth's interior

How does speed affect the friction between a road and a skidding tire?

Speed has no effect on the friction.

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 gain of potential energy.

Where is the center of mass of a baseball? Where is its center of gravity?

The center of mass and the center of gravity are both at the center of the spherical ball.

What is the source of energy in sunshine?

Thermonuclear fusion energy

How do clockwise and counterclockwise torques compare when a system is balanced?

They are equal in magnitude and opposite in direction

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

by a factor of four

Energy cannot be ________________.

destroyed

When Joshua brakes his speeding bicycle to a stop, kinetic energy is transformed to

heat

A circus diver drops from a high pole into water far below. When he is halfway down

his potential energy is halved he has gained an amount of kinetic energy equal to half its initial potential energy his kinetic energy and potential energy are equal

When the speed of a moving car is doubled, how much more kinetic energy does it have?

it has four times as much

When a drawn bow of potential energy 40 J is fired, the arrow will ideally have a kinetic energy

of 40 J

Fossil fuels, hydroelectric power, and wind power ultimately get their energy from ________________.

the sun