chapter 7 - homework questions

A force of 50 N is applied to the end of a lever, which is moved a certain distance. If the other end of the lever moves one-third as far, how much force does it exert?

150 N

A melon is projected into the air with 100 J of kinetic energy in the presence of air resistance. When it returns to its initial level its kinetic energy is

less than 100 j

An object has gravitational potential energy due to its

location

A primary difference between momentum and kinetic energy is

momenta can cancel; kinetic energy cannot.

A light aluminum ball and a heavy lead ball of the same size roll down an incline. When they are halfway down the incline, they will have identical

momentum. kinetic energies. inertias. ((none of the above))

A 1-kg ball dropped from 2 m rebounds only 1.5 m after hitting the ground. The amount of energy converted to heat is about

more than 2.0 J.

If a Ping-Pong ball and a golf ball both move in the same direction with the same amount of kinetic energy, the speed of the Ping-Pong ball must be

more than the golf ball

An object that has kinetic energy must be

moving

A bicycle that travels four as fast as another when braking to a stop will skid

sixteen times as far.

Which requires the most amount of work by the brakes of a car?

slowing down from 100 km/h to 70 km/h

Hydro and wind power are indirect forms of

solar energy

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?

(a) You do F d = 100 N 10 m = 1000 J of work. (b) Because of friction, net work on the crate is less. KE = net work = net force distance = (100 N - 70 N)(10 m) = 300 J (c) So the rest, 700 J, goes into heating the crate and floor. Answer is KE= 300 J

About 40 J is required to push a crate 4 m across a floor. If the push is in the same direction as the motion of the crate, the force on the crate is about

10 N

What will the kinetic energy of a pile driver ram be if it starts from rest and undergoes a 10 kJ decrease in potential energy?

10 kJ

A machine puts out 100 watts of power for every 1000 watts put into it. The efficiency of the machine is

10%

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

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

100%

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

2 W

A 2-kg box of taffy candy has 40 J of potential energy relative to the ground. Its height above the ground is

2 m

A hydraulic jack is used to lift objects such as automobiles. If the input force is 200 N over a distance of 1 meter, the output force over a distance of 0.1 meter is ideally

2000 N

A 2500-N pile-driver ram falls 10 m and drives a post 0.1 m into the ground. The average impact force on the ram is

250,000 N

One kilowatt-hour represents

3.6 million joules

A toy elevator is raised from the ground floor to the second floor in 20 seconds. The power needed using 1000 J of work, is

50 w

The power required to exert 4-N force over 3 meters in 2 seconds is

6 w

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%

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 2-kg ball is held 4 m above the ground. Relative to the ground its potential energy is

80 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 2 m or lifting a 25-kg sack a vertical distance of 4 m?

Both take the same 1000 J. (Work = FD = mg X distance = 10mD)

If a machine multiplies force by a factor of 4, what other quantity is diminished, and by how much?

Distance is diminished to one-quarter.

Exactly what is it that enables an object to do work?

Energy

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

Force and distance, but not energy

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

It has four times as much Kinetic energy = ((1/2)mass X (velocity squared))

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. (Kinetic energy of car = 1/2 X mass x (change in velocity)^2 = Work to stop it = Work in Joules.)

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. (power = (workdone) /Tjme interval) = unit in watts)

Can we correctly say that hydrogen is a new source of energy? Why or why not?

No. It takes energy to make hydrogen.

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

What is recycled energy?

Reused energy that otherwise would be wasted

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

Speed has no effect on the friction.

What is the ultimate source of the energy from fossil fuels, dams, and windmills?

The Sun

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.

What is the source of energy in sunshine?

Thermonuclear fusion energy

Select the correct equations that show that 32 J of work is done when a 4.0-kg block of ice is moved from rest to a speed of 4.0 m/s.

W=mv2/2=((4.0kg)⋅(4.0m/s)2)/2=32J

When is the potential energy of something significant?

When the potential energy changes

A force sets an object in motion. When the force is multiplied by the time of its application, we call the quantity impulse, and an impulse changes the momentum of that object. What do we call the quantity force multiplied by distance?

Work

How much work is done on a satellite in a circular orbit about Earth?

Zero

A machine that promises more energy output than input is

a fantasy

What is the unit of work?

joule

Acrobat Bart at the circus drops vertically onto the end of a see-saw, with his partner Art equidistant from the fulcrum at the other end. Art is propelled straight upward a distance twice that of Bart's dropping distance. Neglecting inefficiencies we see

art has half the mass of bart

The bob of a simple pendulum has its maximum kinetic energy at the

bottom of its swing

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

by a factor of four

Impulse involves the time that a force acts, whereas work involves the

distance that a force acts

The unit kilowatt-hour is a unit of

energy

A moving object has

energy. momentum. velocity. speed. ((all of these))

Two identical arrows, one with twice the speed of the other, are fired into a bale of hay. Compared with penetration of the slow arrow, the faster arrow penetrates

four times as far

The work you do when pushing a shopping cart twice as far while applying twice the force is

four times as much

Two identical golf carts move at different speeds. The faster cart has twice the speed and therefore has

four times the kinetic energy.

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

four times.

Which of these is not a form of solar energy?

geothermal energy

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

heat

No work is done by gravity on a bowling ball that rolls along a bowling alley because

the force on the ball is at right angles to the ball's motion.

Both a 50-kg sack is lifted 2 meters from the ground and a 25-kg sack is lifted 4 meters in the same time. The power expended in raising the 50-kg sack is

the same

After rolling halfway down an incline, a marble's kinetic energy is

the same as its potential energy

Earth's primary energy source is

the sun

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

the sun

A ball rolling down an incline has its maximum potential energy at

the top

Strictly speaking, more fuel is consumed by your car if the air conditioner, headlights, or even a radio is turned on. This statement is

true

Two identical arrows, one with twice the kinetic energy of the other, are fired into a bale of hay. Compared with penetration of the slow arrow, the faster arrow penetrates

twice as far

A crate of grapes lifted 10 meters gains 200 J of potential energy. If the same crate is instead lifted 20 meters, its gain in potential energy is

twice as much

A 1000-kg car and a 2000-kg car are hoisted to the same height. Raising the more massive car requires

twice as much work.