Goal: To understand EnergyObjectives:

1) To learn about What energy is2) To learn about Work3) To learn about Power

4) To understand the relationships and differences between Potential and Kinetic energy

5) To learn about the different Forms of energy6) To understand the relationships between Work and

Kinetic energy7) To understand theTransfer of energy

8) To understand the uses for Energy and our world – present and future

What is energy?

• Energy is what is needed to do stuff.• Energy is required to heat.• Energy is required to power an AC to cool.• Energy is needed to move things• Energy is needed to build things• Energy lights our lights and powers our

TVs.• Energy drives our cars.

Work

• One way to measure the use of energy is by measuring work (work is an energy).

• Work = Force * distance• Net Work = Net Force * Net distance

• Units of Work/Energy:• Work = Force * distance = Newton * m• Newton * m = Joule

Other Work/Energy Units

• Watt-hour

• Kilowatt-hour (more on these later)

• BTU (British Thermal Unit)

• Therm (= 100,000 BTUs)

• Calories

• Mostly we will use Joules

Example:

• You walk up a set of stairs. Lets assume your weight is 700 N. If the top stair is 3 m higher than the bottom stair then how much work have you done on yourself?

Batman

• Batman slides across the lake at a constant velocity.

• If he travels 800 m horizontally and has weight of 900 N then what work has been done on Batman?

Dropping the ball

• You want to sit on the couch but for some reason a bowling ball of mass 7 kg is sitting on the couch right where you want to sit.

• Throwing the bowling ball would be a bad idea, so you pick up the bowling ball, lift it 0.2 m above the couch seat and then gently set it onto the floor.

• If the floor is 0.3 m below the couch seat then what is the total work done on the bowling ball by you?

• What is the total work done by the earth?

Push start

• Ignoring friction, you push your 600 kg car with a force of 2000 N for a distance of 10 m.

• How much work have you done on the car?

• Where is this work going to go?

Power

• Power is quite simply the rate at which you do work.

• Power = Work / time = Energy / time

• So, the faster you do work, the more power you are providing.

• Units: Power = Energy / time = J/s• J/s = Watts

Power example

• A forklift which generates a force of 2500 N lifts a 225 kg bundle up a distance of 1 m in a time of 2 seconds.

• What is the work done by the forklift?

• What is the power generated by the forklift?

Lights out

• An incandescent light bulb has a power of 60 Watts.

• If you turn the light bulb on for 10 hours how much energy does the light bulb use?

• NOTE 1 Watt * 1 hour = 1 watt-hour of energy.

• If 1000 Watt-hours costs you $0.10 then what is the daily cost of a single light bulb?

• What is the yearly cost of this single light bulb?

Fluorescent bulbs

• For the same light, they use 4 times less power!

• How much money could 1 Fluorescent bulb save you in 1 year?

• If you have 10 bulbs to replace how much do you save per year by going Fluorescent and how does that compare to the $3 each that the bulbs cost?

Potential and Kinetic Energy

• All energies are comprised of Potential and Kinetic Energies.

• Kinetic Energy = energy of motion

• Potential Energy = stored energy

Potential Energies:

• Spring potential (Mechanical Energy)

• Chemical potential

• Gravitational potential

• Electromagnetic potential

• Nuclear potential

• Mass Energy

Kinetic Energy

• Moving energy

• Heat energy

• Light Energy

• Wave energy

• Sound energy

Work vs. Kinetic energy

• Work = Force * distance• Force = mass * acceleration• Distance = ½ acceleration * time * time

• So,• Work = mass * acc * ½ acc * time * time• Acceleration * time = velocity

• Therefore,• Work = ½ mass * velocity * velocity• Notice that the above is the equation for Kinetic energy!

Lets prove it!

• You push a box across a frictionless floor.• You apply a 300 N force to the 20 kg box.• You push the box for 5 m.• What is the work you have done to the box?• What is the acceleration on the box?• How long does it take to push the box 5 m (we

have distance and acceleration…)?• Using v = at, what velocity is the box traveling

when you get to the 5 m mark.• Now, find the Kinetic energy of the box

(KE = ½ mass * velocity * velocity)

Energy transfers

• Energy is not created or destroyed, but it is always moving from one form to another.

• Lets examine gravitational potential transfers to kinetic energy and back.

Gravitational Potential Energy

• Work = Force * distance

• What force do you need to overcome gravity?

Gravitational Potential Energy

• Work = Force * distance• What force do you need to overcome

gravity?

• Work = mass * gravity * distance• The distance is the height,• So, Work = mass * gravity * height• This is called Gravitational Potential

Energy

Example

• You are holding a 2 kg ball at a height of 0.6 m. What is the Gravitational potential energy of the ball?

• You drop the ball. What happens? By how much will the gravitational potential of the ball change?

Example• You are holding a 2 kg ball at a height of 0.6 m. What is

the Gravitational potential energy of the ball?• You drop the ball. What happens? By how much will

the gravitational potential of the ball change?

• Lets find out what the kinetic energy does.• Find the time it takes to fall 0.6 m using gravity and the

distance.• Find the velocity the ball hits the ground at.• Find the kinetic energy of the ball when the ball hits the

ground.

• How does the kinetic energy of the ball compare to the change in gravitational potential?

Conclusion

• We have seen how energy is used, transferred between forms, and why it is useful.

• We have discovered how to find work, power, kinetic energy, and potential energy.

• What we have not done is take a look at how we actually use energy as a nation now, and how and why we should make a change in the future.