Chap. 7 - Physics

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

2 W

One kilowatt-hour represents

3.6 million joules.

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

The work done in pushing a TV set a distance of 2 m with an average force of 20 N is

40 J

When the useful energy output of a simple machine is 100 J, and the total energy input is 200 J, the efficiency is _______.

50%

Calculate the work done in lifting a 400-N barbell 1.4m above the floor.

560 J

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

6 W

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.

Why does the bowling ball behave the way it does the first time Dr. Hewitt lifts the bowling ball near his teeth and lets go?

All of the initial energy of the ball was converted completely back to potential energy when the ball returned.

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.

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.

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

It has four times as much.

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.

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

The Sun

What happens the second time Dr. Hewitt lifts the bowling ball near his teeth and gives it a push?

The ball leaves Dr. Hewitt and returns to him, going past the point where it was released.

What happens the first time Dr. Hewitt lifts the bowling ball near his teeth and lets go?

The ball returns to Dr. Hewitt, stopping almost exactly at the point where it was released.

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.

Why does the bowling ball behave as it does when Dr. Hewitt lifts it and gives it a push?

The extra energy from the push is converted into kinetic energy, which is then converted into more potential energy at the end of the motion than the ball had when it was released.

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

bottom of its swing.

Energy cannot be _______.

destroyed

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

Two 2-m/s pool balls roll toward each other and collide. Suppose after bouncing apart each moves at 4 m/s. This collision violates the conservation of

energy (kinetic)

Newton's Cradle consists of an aligned row of identical elastic balls suspended by strings so they barely touch one another. When two balls are raised from one end and released, they strike the row and two balls pop out from the other end. If instead, one ball popped out with twice the speed of the two, this would violate

energy conservation.

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.

A block of ice sliding down an incline has half its maximum kinetic energy

halfway down.

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

heat

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

inertias. kinetic energies. momentum. potential energies. NONE OF THE ABOVE

What is the unit of work?

joule

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

When you and your snowboard are in motion, which of the following can be zero?

mass momentum inertia kinetic energy NONE OF THE ABOVE

If the speed of a motor scooter doubles, which of the following also doubles?

momentum

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

Although perpetual motion is the natural order of things in the universe

no perpetual-motion device that multiplies energy exists.

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

the bottom

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.

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.

Relative to an initial height, an object raised twice as high has a gravitational potential energy

twice as much.

Emily holds a banana of mass m over the edge of a bridge of height h. She drops the banana and it falls to the river below.

v = square root of 2gh

A moving object has

velocity. energy. momentum. speed. ALL OF THESE

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

What is the gain of potential energy of the barbell when it is lifted to this height?

ΔU = 560 J

Which has greater kinetic energy?

a car of half the mass traveling at 60 km/hr

A machine that promises more energy output than input is

a fantasy.

A) How much work is done when you push a crate horizontally with 110N across a 5.0-m factory floor? B) If the force of friction on the crate is a steady 70 N, find the KE gained by the crate. C) Find the energy turned into heat.

a) 550 J b) 200 J c) 350 J

The roller coaster ride starts from rest at point A. a) Rank these quantities from greatest to least at each point. b) Rank KE from greatest to least at each point. c) Rank PE from greatest to least at each point.

a) D, B, C, E, A b) D, B, C, E, A c) A, E, C, B, D

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

Neglecting air resistance, Sammy Smarts on a high ladder releases a ball that strikes the ground with 100 J of kinetic energy. If he were to instead throw the ball straight upward, it will soon reach the ground with a kinetic energy of

100 J

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

Nellie Newton applies a force of 50N to the end of a lever, which is moved a certain distance. If the other end of the lever moves one-third as far, find the force it exerts.

150 N