Work and Power Notes

What Work and Power are not… Work is not where you go to earn a check! Power is not how strong you are!

Work is a transfer of energy that occurs when a force makes an object move in a direction parallel to and opposite the force.

Doing work on an objectgives it energy!

Work-Energy Theorem

Is work being done?

What 2 conditions must be present in order for work to be done?

(1) there must be a force

(2) there must be movement or motion

Formula for work:

W = F d(Joules) (Newtons)(meters)

W = F d (cosθ)

Work and Energy are both measured in Joules because they are related!

Power

Amount of work done in a certain amount of

time. (i.e. how fast you do work).

Formula P (Watts) = W (Joules)

t (seconds)

Roller Coaster Connection Work is done on the roller coaster when it is

lifted by the lift chain (machine) to the top of the first hill.

That is how it is given potential energy! If the lift chain is faster, it is more powerful.

Energy

Mechanical Energy Energy is the ability to do work or make

something change. Energy is measured in Joules (J). Two main types: potential and kinetic

Potential Energy Energy of position (stored energy) Gravitational potential energy (GPE)

is due to how high something is .GPE = mgh

Chemical potential energy is due to the position of charges (includes nuclear).

Elastic potential energy is due to the position of something stretchy.

Kinetic Energy

Energy of motion, how fast it is going Almost all other forms of energy are due to

the motion of something: electrical (electrons), solar(photons/waves), light/radiant (photons/waves), thermal (molecules), sound (molecules)

KE = 1/2mv2

Law of Conservation of Energy Energy can’t be created or

destroyed, but it can change form. Sometimes we say energy is

“LOST” but it is not lost, it is converted to something else less useful (heat)

Formula

KEi + PEi = KEf + PEf

½mv²i + mghi = ½mv²f + mghf

Roller Coaster Connection

The ball has the greatest potential energy (PE = mgh) at the top of the first (highest) hill.

It is not moving there so it has no kinetic energy.

Roller Coaster Connection - Conservation

Kinetic energy (KE = 1/2mv2) is at its highest when the roller coaster is the fastest.

This occurs at the lowest point of the roller coaster because that is where all PE is converted to KE (there is no potential energy at that point h=0)

Of course in the real world…

Both air resistance and friction act on the roller coaster, transforming some of the kinetic energy to thermal energy (heat).

Designing a roller coaster: What force drives a roller coaster? What keeps a roller coaster going? Why does the first hill for a roller coaster

have to be the highest? Where is the only place work is done on a

roller coaster (except to stop it)? How can you increase the power on a

roller coaster?

Energy Transformations

Station 1

Food contains chemical energy which we turn into kinetic energy.

We eat food to obtain energy from the bonds.

Station 2

The rubber band will go flying if you stretch and then release it.

This is a conversion of elastic potential energy to kinetic energy.

Station 3 The rubber ball has

gravitational potential energy when you hold the ball.

Gravitational PE is converted to kinetic energy when you drop it.

Station 4

Radiant/light energy from the light bulb is converted into kinetic energy to make the flags of the radiometer spin.

The light bulb also converts electrical energy from the outlet to radiant and thermal energy.

Station 5

The mechanical energy of spinning the hand generator handle is converted to electrical energy.

The electrical energy is then converted to radiant and thermal energy, lighting the light bulb.

Station 6Chemical energy in the battery is converted to electrical energy that travels down the wire and is converted to kinetic energy when the handle of the hand generator spins.

Station 7The chemical energy of the gasoline is converted to thermal energy and kinetic energy as the car moves around the track.

Station 8

Chemical Energy in the lemon is converted to electrical energy.

Station 9

Radiant energy is converted to electrical energy to power the calculator.

Station 10

The roller coaster’s gravitational potential energy at the top of the hill is converted to kinetic energy on the way down.