Physics ISouth Oldham HS

Unit 2: EnergyPart 2: Types of EnergyConservation of EnergyWork-Energy Theorum

EnergyDefinition: the ability to do workUnits: just as in work, Joules (J)

Mechanical EnergyDefinition: energy due to position or the

movement of somethingTwo types:

Kinetic energyPotential energy

Another way of looking at it: the sum of kinetic and potential energy

Examples…An archer pulls back on a bowstring,

allowing it to do work on the arrow.A pile driver is raised, allowing the ram to do

work on an object when it falls.A spring is wound, allowing it to do work on

gears that run a clock.In each of those cases, something has been

acquired that enables the object to do work.

Potential Energy (PE)Definition: energy that is stored and held in

readinessAn object that has potential energy has the

potential for doing work.When a bow string is drawn back, energy is

stored in the string because the bow can do work on the arrow.

Chemical energy in fuels are examples of potential energy. This includes food!

Gravitational Potential Energy (GPE)Definition: potential energy due to elevated positionExamples: water in a reservoir, the ram of a pile

driverEquations:

GPE= weight x heightGPE=mass x acc due to gravity x heightGPE=mgh

Remember that weight = mg, so if you are given the weight (in N), then to find GPE, multiply weight x height

How are the variables in the equation related?

Questions Part 11. How much work is done on a 100 N boulder

that you carry horizontally across a 10-m floor?

2. Regarding #1, how much PE does it gain?3. How much work is done on a 100 N boulder

when you lift it 1 m?4. What power is expended if you lift the

boulder a distance of 1 m in a time of 1 s?5. What is the gravitational potential energy of

the boulder in that lifted position?

Questions Part 11. How much work is done on a 100 N boulder

that you carry horizontally across a 10-m floor? Zero

2. Regarding #1, how much PE does it gain? Zero

3. How much work is done on a 100 N boulder when you lift it 1 m? 100 J

4. What power is expended if you lift the boulder a distance of 1 m in a time of 1 s? 100 W

5. What is the gravitational potential energy of the boulder in that lifted position? 100 J relative to its starting position

The potential energy of the 100-N boulder with respect to the ground below is 200 J in each case.

a. The boulder is lifted with 100 N of force.b. The boulder is pushed up the 4-m incline with 50 N of force.c. The boulder is lifted with 100 N of force up each 0.5-m stair. What does this tell us?

Potential Energy

Kinetic Energy (KE)Definition: energy of motionEquation:

mv2

How are the variables related?KE and m = direct relationshipKE and v = quadratic relationship

Work-Energy TheorumIt’s so simple!Here it is….Whenever work is done, energy changes. So…the change in work is equal to the

change in energy.

Interesting note…We can combine the work and KE equationsW=Fdmv2

Since KE=W, then Fd= mv2

Using that equation, in a car, what happens to your stopping distance if you are going twice as fast?

It quadruples!

Conservation of EnergyEnergy changes forms. It transforms.In using a sling shot, you do work when you pull the

rubber band back. Your work gives the rubber band potential energy. When you let go, that stone in the sling shot flies

through the air – its potential energy has been converted into kinetic energy.

If the stone hits something, say, a fence post, it will make a sound (a type of kinetic energy) and warm up the stone and the fence post (another type of kinetic energy).

Law of Conservation of EnergyEnergy can neither be created or destroyed.

It can be transformed from one form to another, but the total amount of energy never changes.

Total energy stays the same.

Due to friction, energy is transferred both into the floor and into the tire when the bicycle skids to a stop.

a. An infrared camera reveals the heated tire track on the floor.

Conservation of Energy and the Work-Energy Theorem