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### Energy

### ENERGY

Kinetic and potential

Standard: S8P2

a.Explain energy transformation in terms of the Law of Conservation of Energy.

b. Explain the relationship between potential and kinetic energy.

Energy can be classified as potential or kinetic

- Potential energy: energy of position

The boulder has

more gravitational

potential energy when

measured from point A

compared to B.

PE = mgh

PE = mgh

- m = mass in kilograms
- g = acceleration due to gravity
(9.8 m/s2)

- h = height in meters
- Potential Energy is measured in
kg • m/s2• m =

newton meter =

Joules

Example 1:

- If the boulder has a mass of 8kg, and distance B is 10m, what is the potential energy of the boulder relative to the plateau?

8kg

10m

Example 2:

- If distance A is 20m, what is the potential energy of the boulder relative to the bottom?

8kg

20m

Example 3

- When work is done to stretch something, it is called elastic potential energy.

Find the elastic potential energy stored in a drawn bow if it takes an average force of 100N to pull the arrow back a distance of 0.5 meters.

Kinetic energy

- Energy of motion
- KE = ½ mv2

KE = 1/2mv2

- m = mass in kilograms
- v = velocity in meters/sec
- Kinetic energy is measured in
- kg • m/s • m/s =
- kg m/s2 • m =
- newton meter =
- Joules

Example 4

- The bird has a mass of 2 kg. It is flying at a speed of 5 m/s. Find its kinetic energy.
- KE = ½ mv2
- = ½ • 2kg • (5m/s)2
- = ½ • 2kg • 25 m2/s2
- = 25 joules

Kinetic and potential energy conversions

- Describe the energy conversions in this picture:
- At the top:
- ¼ of the way down:
- ½ way down:
- ¾ down
- At the bottom:
- Is the sum of KE + PE a constant?

All PE, no KE

3/4 PE, 1/4 KE

1/2 PE, 1/2 KE

1/4 PE, 3/4 KE

No PE, All KE

Yes! It’s always

10,000 J in this case.

Kinetic and potential energy convert to one another

PE max

PE max

no KE

no KE

KE max

KE max

no PE

no PE

E=mc2

- E = energy in joules
- m = mass in kilograms
- c = speed of light
- (3 x 108 m/s)

Example:

- How much energy can be given off by 5 grams of mass? ( 10 paper clips = 5 g)
- 5g = 0.005 kg
- E=mc2
- = 0.005kg x (3 x 108 m/s)2
- = 0.005 kg x 9 x 1016 m2/s2
- = 0.045 x 1016
- = 450,000,000,000,000 J

WOW!!!

- 450,000,000,000,000 J is the amount of energy in 3 million gallons of gasoline!

The End

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