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Energy

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Energy

Kinetic and potential

ENERGY

Standard: S8P2

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

b. Explain the relationship between potential and kinetic energy.

- Potential energy: energy of position

The boulder has

more gravitational

potential energy when

measured from point A

compared to B.

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

- 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

- PE = mgh
- = 8kg • 9.8m/s2 • 10m
- =784 J

8kg

10m

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

8kg

20m

- PE = mgh
- = 8kg • 9.8m/s2 • 20m
- = 1568 J

8kg

20m

- 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.

- PE = mgh = Fd

Fxd = 100N x 0.5m

= 50J

- Energy of motion
- KE = ½ mv2

- 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

- 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

- 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.

PE

and KE

Max PE

no KE

Max KE

No PE

Max PE

No KE

What happens when the cord is cut?

Potential energy

is converted to

kinetic energy!

PE max

PE max

no KE

no KE

KE max

KE max

no PE

no PE

- Albert gave us the answer!

E=mc2

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

- 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

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

Einstein's theories of

relativity have been

proved during solar eclipses.

Light is bent by

gravity

See ya!

The End