Chapter 9: Energy

How would you double the work that is done on an object?

double the mass of the object

In physics, work is defined as

force times distance

If Nellie Newton pushes an object with twice the force for twice the distance, she does

four times the work

What happens to the power if the time is doubled?

halved

When a car's speed triples, its kinetic energy

increases by nine times

The unit of work is the

joule

An object that is moving must have

kinetic energy

Potential energy is the energy of an object has because of its

location

What do we call the total of potential and kinetic energy in a system?

mechanical energy

An object that has kinetic energy must be

moving

Kinetic energy of an object is equal to

one half the product of its mass times its speed squared

What is the energy that an object has because of its location?

potential

The amount of work done on an object in order to lift it up is equal to the

potential energy you give the object

A job is done slowly, and an identical job is done quickly. Both jobs require the same amount of work but different amounts of

power

What is the work done on an object divided by the time?

power

Which has greater kinetic energy, a car traveling at 30 km/h or a half-as-massive car traveling at 60 km/h?

the 60 km/h car

The amount of potential energy possessed by an elevated object is equal to

the work done in lifting it

Watt is the unit of power.

true

If you lift two loads up one story, how much work do you do compared to lifting just one load up one story?

twice as much

In physics, what is the force applied by the distance moved?

work

Power is defined as the

work done on an object divided by the time taken to do that work

Simple machines all work on the principle that

work input equals work output

You walk up the stairs. Your friend (exact same weight) runs up the stairs. Which is true?

your friend exerts more power

Disadvantages to using a lever

- Can only lift objects a short distance

Disadvantages to using a pulley

- Difficult to construct. Usually complicated arrangement of ropes.

Advantages to using an inclined plane

- Easy to create - Can move objects long distances

Disadvantages to using an inclined plane

- Lots of friction (low efficiency) - Usually takes a lot of room

Advantages to using a pulley

- Multiple forces in small areas - Very little friction (more than levers, less than inclined planes)

Advantages to using a lever

- Very little friction - Very simple to create

A rock is dropped off a cliff. What is true about its energy?

- at the top, it is all potential - at the bottom, it is all kinetic - total energy stays constant

What is an example of a simple machine?

- pulley - inclined plane - lever - screw

You carry a 30 N bowling ball 10 m across a room. How much work did you do on the ball?

0 J Has to be done in the same direction

Divine mechanical advantage and efficiency as they apply to simple machines.

Mechanical Advantage is the number of times your force is multiplied by a simple machine. It is the output force divided by the input force. The higher the MA, the more useful your simple machine. Efficiency is the percent of useful work you put into a simple machine. The higher the efficiency, the greater the amount of input work is converted into moving the object. Friction lowers the efficiency

What is the law of conservation of energy? Explain the idea using a boy sledding down a hill. Where is the potential energy the greatest? Where is the kinetic energy the greatest? Where is the mechanical energy the greatest?

The Law of Conservation of Energy states that energy cannot be created or destroyed: it can only change forms. For a pendulum, the total amount of energy is constant, it simply converts between kinetic and potential. At the top of the pendulum's swing, the energy is all potential, none is kinetic. At the bottom, it is all kinetic, none is potential. The pendulum eventually stops swinging because of friction, as energy is converted into heat.

The unit of power is the

Watt

Explain the difference between work and power. Use as an example the lab we did running and walking up the stairs.

Work is equal to the Force times distance while power is the work divided by time (or the rate that work is done). Work is measured in Joules, Power in Watts. Whether running or walking up the stairs, you do the same amount of work, since it is independent of time. Running up the stairs requires more power, because your time to complete the work is less, or the rate you are doing work is more.

You roll a ball and give it 100 J of kinetic energy. How much work did you do?

100 J

What happens to the kinetic energy of a car if the speed quadruples?

16x as much

A sling shot is pulled back so it has 200 J of PE. When released, how much KE does it have?

200 J

What happens to work done on an object if you triple the force and triple the distance moved?

9x the work

3 identical objects are thrown at the same speed. 1 straight up, 1 straight out, 1 straight down.

all hit the ground at the same speed.

Mechanical energy can be in the form of

both kinetic and potential energy

Which requires more work: lifting a 70 kg sack vertically 2 meters or lifting a 35 kg sack vertically 4 meters?

both require the same amount of work

Which requires more work? Lifting a 100 N sack 4 m or lifting a 200 N sack 2 M?

both the same