PY 131 Chapter 7 Conceptual Academy

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

four times as much.

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

the same.

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

four times.

When traveling twice as fast your kinetic energy is increased

by four.

Energy cannot be

destroyed

In a simple machine, how much work is done when an input of 10 N acts over a distance of 5 m?

50 J.

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

50%

The single thing that all living things require is

a source of energy

Work is force times

distance

Work is best expressed as

Fd

Gravitational potential energy is best expressed as

mgh (mass x acceleration due to gravity x height)

Relative to the floor, a block higher on a ramp has increased

potential energy

In this lesson kinetic energy is derived by way of

Newton's second law.

To say a block of ice on a ramp has a given potential energy, means energy relative to

the height of the block above zero ground level.

A block sliding down a ramp gains KE equal to

the decrease in its PE.

The work-energy theorem states that the work done on an object is equal to its

change in kinetic energy.

Since we're looking for distance Nellie skids, we begin our solution by writing

d=

In this problem, the mass of Nellie and her bike

cancel in the solution, so is not relevant.

The swinging bowling ball loses energy in the form of

heat

A swinging bowling ball has the most kinetic energy at the

bottom of its swing.

Distance times force provides

work

A little girl lifts a heavy truck by applying a small force over a

large distance.

How much work did Tiger Ted perform in pulling 5m of rope at 50N?

250 J

(work: force x distance)

An efficiency of 100 percent is not possible because of

heat loss

Electricity is a

carrier of energy

A post is driven 0.2 meters into the ground by a 5000 N weight falling 20 meters before contact creating a force of impact (on average) of

500,000 N.

Depressing the input piston of a hydraulic press by 20 centimeters lifts the output piston by 1 centimeter. Thus, inputting a force of 1 N can raise a load with a maximum weight of

20 N

To push with 1 Newton of force across 1 meter in 1 second requires a power of

1 W

Cranking out 100 Watts for every 1000 Watts of power received, a machine's efficiency is

10%

Having rolled half way down an inclined plane, a bowling ball's kinetic energy is

the same as its potential energy

Pushing an object a certain distance, while exerting two times the force, you do

twice as much work

Bullets are fired forward from an old World War I airplane. The airplane's momentum is thus

decreased

The most important property determining an object's potential energy is its

location

On a straight and level track, a freight car moving westward at 1 m/s collides head-on and couples with an identical second freight car moving eastward at 2 m/s. Together they roll

eastward

When pushing an object two times the distance while exerting a force that is the same, you do

twice as much work

To slow a car down, the car's brakes must to work. Consider slowing down from 100 km/h to 70 km/h and compare that to slowing down from 70 km/h to a stop. For the brakes, which requires more work?

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

A rock raised two times as high has a potential energy that is

twice as much

A workman can lift a barrel 1 meter vertically or roll the barrel up a 2 meter ramp to the same height. Using the ramp, the force needed is about

half as much

When pushing against the wall of a brick building for 30 minutes or 60 minutes

no work on the wall is done in either case

Gravity performs no work on a 5 kg bowling ball rolling down a bowling lane because

its force causes no change in distance

Within a vacuum, a feather and a nail descend with the same

accelerations

To lift a 100 kg bag 2 meters, compared to lifting a 50 kg back 4 meters, within the same amount of time requires more power or less power?

the same power

Five identical steel balls each hanging from a string are lined up at the same height and in a row and in contact with each other. Two balls on one end are together lifted and swung at the remaining three. Two balls on the opposite side pop out. If instead one ball popped out to twice the height, this would violate the conservation of

energy

A rifle recoils as a bullet is fired with equal

but opposite amounts of momentum

Can a car jack multiply the input of force?

always true

Drawing a bow so that it has 40 joules of potential energy means that an arrow fired from this bow ideally will have a kinetic energy that's

40 J

In deep space, two identical 5000 kg asteroids glide toward each other, one at 1 m/s and the other at 2 m/s. Just before colliding, their total kinetic energy is

12,500 J

A barrel rolling down a hill has its greatest potential energy at

the top

An arrow shot with double the kinetic energy of another identical arrow will penetrate into the same bale of hay

twice as deep

Within 50 seconds a woman does 100 joules of work. Her power output is

2 w

A car jack, much like an inclined plane, can increase energy

never ever

As the velocity of a moving ball doubles, there is also a doubling of

momentum

Each rolling at 3 m/s, two billiard balls collide head on and then bounce away from each other at 6 m/s. However, this violates the conservation of

energy

While traveling down a highway, a car's engine works against a resistance force averaging 500 Newtons. The energy to do this comes from the gasoline, which holds 40 million joules in each liter. So, for every liter of gasoline, how many kilometers can the car travel assuming it works with 100 percent efficiency?

80 kilometers

A train carrying hundreds of open and empty hot tubs rolls horizontally with no friction in a heavy rain. As water collects in the tubs, the train's speed

decreases

A car brakes to a complete stop. This transforms its kinetic energy into

heat

A bowling ball at rest is set into motion upon being hit head-on by a flying golf ball, which bounces backward. Compare the momenta and kinetic energies of the two balls after the collision. The bowling ball has

more momentum, but less kinetic energy

A truck traveling four times as fast as a slower-moving identical truck has

16 times the KE

It took 1000 joules to lift a crate 10 meters in 20 seconds. The amount of power required to do this was

50 W

How much work is required to push a 15 kg chair with a force of 20 N over a distance of 2 meters?

40 j

A 100 kg motorcycle rolls slowly off a cliff's edge, falling for 3 seconds to a garbage dump below. Just before hitting the ground its momentum is

3000 kg m/s

An ice skater carries a log across a frozen pond at a constant speed. The amount of work done to the log is

zero

For a task requiring two times the time and two times the work, the power needed is

unchanged

A plastic ball and a lead ball of identical size roll down an inclined plane. Half way down they have identical

none

To lift a 100 kg bag 2 meters, compared to lifting a 50 kg back 4 meters, requires more work or less work?

\n **The same amount of work.**

Impulse concerns force and time, while work concerns the

distance that a force acts

A rock is thrown straight upward with 200 joules of kinetic energy. Considering air resistance, by the time it falls back to the level at which it was thrown its kinetic energy is

less than 200 J

How many Joules of energy are in one kilowatt-hour? In two kilowatt-hours there is energy. In units of joules, how much?

7\.2 million joules

On a straight and level track with no friction, a 5000 kg freight car moving westward at 1 m/s collides head-on and couples with an identical 5000 kg freight car moving eastward at 2 m/s. Together they roll with a combined momentum of

5000 kg m/s

Performing an amount of work in one third the time requires a power output of

three times the usual power output

In deep space, two identical sticky globs of taffy are heading directly toward each other, one at 2 m/s and the other at 1 m/s. After their head-on collision, their combined speed is

0\.5 m/s

A rifle recoils from firing a bullet, which then has greater kinetic energy than the rifle because the force on the bullet was acting over a

longer distance

A golf ball and a Ping Pong ball have the same kinetic energy as they move in the same direction. The Ping Pong ball's speed must be

more than the golf ball

A 2000 kg truck and a 4000 kg truck are lifted the same distance, but lifting the heavier truck requires

twice as much work

To have kinetic energy, an object must be

moving

A truck moves at 30 km/hour while another truck of half the mass moves at 60 km/hour. The one with greater kinetic energy is the

faster moving truck

Neglecting air resistance, a skier racing straight down a slope has her greatest speed at

the bottom

A box raised 10 meters receives 200 joules of potential energy. If lifted 20 meters, the potential energy gain is

twice as much

Once in motion, an object can have a zero amount of

none of these

As it rolls down an inclined plane, a marble has its least speed

near the top of the incline

A beam is being driven into the ground by a pile driver. How far the beam gets pushed into the ground with each strike will depend upon

any of these choices

With the brakes locked, a truck traveling at 50 km/hour skids to a stop in 20 meters. If it had been moving at 150 km/hour, the skid distance would have been about

180 m

In a vaccum, a light and heavy ball dropped from the same height will have the same

**accelerations.**

Stepping off a board 10 meters above a swimming pool, a 500 N diver falls into the pool with a kinetic energy of

5000 J

Having kinetic energy, a ball must also have

momentum

Running your car with the headlights turned on causes more fuel to be consumed. Is this true?

This is always true

With a system of pullies, a 100 N crate can be lifted 10 meters with 2000 joules of work input, which gives an efficiency of about

50%

Two identical jobs. One performed slowly. The other performed quickly. Both will require equal work, but different amounts of

power

In deep space, a sticky glob of taffy collides head-on and sticks to an identical glob of taffy at relative rest. After the collision, the total momentum remains the same, but the kinetic energy

decreases

A bunch of rocks are thrown upward, each with a different initial velocity. What quantity is the same for each rock?

acceleration

In skidding to a stop, a truck moving two times as fast as an identical truck will skid

four times as far

Off the edge of a rooftop, a woman releases a rock, which hits the ground with 200 joules of kinetic energy. If she instead tosses the rock straight upward, it would eventually hit the ground with how much kinetic energy? (Ignore air resistance)

more than 200 J

A bullet is fired from a rifle that recoils backward. After the bullet is shot outward, compared to the rifle it has

a greater kinetic energy

From rest and a height of H, a cannon ball of mass M falls to the ground. Its kinetic energy upon hitting the ground

is proportional to H

When a 2 kg ball has 40 joules of potential energy relative to the floor, its height above the floor is about

2m

When at rest, an object can also have

potential energy

When a 2 kg ball is positioned 4 meters above a floor, its potential energy relative to the floor is about

80 J

A rolling ball has

all of these

A barrel rolling down a hill has its greatest kinetic energy at

the bottom

A ball of 1 kilogram and 2 meters above the ground is dropped. It hits the ground and bounces back up to a height of 1.5 meters. About how many joules of energy is lost?

more than 2.0 J