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Calculating Energy

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Calculating Energy

- Total Energy does NOT change!
- The amount of KE, PE and THERMAL change, but always add up to the same total!

THE LAW OF CONSERVATION OF ENERGY:

Energy cannot be created or destroyed, it only changes forms

Potential Energy

Stored Energy

- Gravitational PE
- Determined by an object’s height

- Elastic PE
- Stretching or Compression

- Chemical PE
- Food (not yet eaten)
- Gasoline (not yet burned)
- Battery (not yet used)

- Any time an object has height above a reference point.
- GPE = M*G*H
- Mass (in kilograms)
- G (acceleration due to gravity = 9.8 m/s2)
- Height (in meters)

The UNIT for Energy is JOULE (J)!

- A 0.5-kg squirrel sits at the top of a 10-meter tall tree eating an acorn. How much Gravitational Potential Energy does the squirrel have?

Step 1: Write the given information and the formula

M = 0.5 kg

H = 10 m

G = 9.8 m/s2

GPE = mgh

Step 2: Substitute the given values into the formula

GPE = 0.5kg x 9.8 m/s2x 10 m

Step 3: Solve the problem (with units!)

GPE = 49 Joules

- Monesha drives her 1000-kg Mercedes to the top of a hill 7 meters tall. How much GPE does the Mini have at the top of the hill?
- Karnell throws a 1-kg football straight up in the air. How much GPE does the football have when it is 9 meters high?
- Tierney jumps on a trampoline. If she has a mass of 65 kg and jumps 5 meters into the air, how much GPE can she obtain?

Kinetic Energy

Energy of Motion

- A falling ping pong ball
or

- A falling bowling ball?
- Increasing mass increases KE!

- How do you get your bike to go really really fast down a hill?
- Increasing speed increases KE!

- Its MASS (kg)
and

- Its VELOCITY (m/s)!

KE = ½ * M * V2

The UNIT for Energy is JOULE (J)!

- A 3000 kg delivery truck drives down the street at 15 m/s. How much Kinetic Energy does it have?

Step 1: Write the given information and the formula

M = 3000 kg

V = 15 m/s

KE = 1/2mv2

Step 2: Substitute the given values into the formula

KE = ½ x 3000kg x (15 m/s)2

Step 3: Solve the problem (with units!)

KE = ½ x 3000kg x 225 = 337,500 J

- Jasmine pedals her bike at 20 m/s. If she and her bike have a mass of 100 kg, how much KE does she have?
- A 6-kg alley cat is chasing a mouse at a speed of 4 m/s. How much KE does the cat have?
- Raven kicks a 1.5-kg soccer ball giving it a velocity of 3 m/s. How much KE does the ball have?