Calculating Energy

What did you notice about the TOTAL ENERGY during the skatepark lab?• 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 EnergyStored Energy

Any energy that is STORED UP is Potential 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)

Gravitational PE• 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)!

EXAMPLE 1• 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

Step 2: Substitute the given values into the formula

Step 3: Solve the problem (with units!)

M = 0.5 kgH = 10 mG = 9.8 m/s2

GPE = mgh

GPE = 0.5kg x 9.8 m/s2 x 10 m

GPE = 49 Joules

Practice on Your Own1. 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?

2. Karnell throws a 1-kg football straight up in the air. How much GPE does the football have when it is 9 meters high?

3. 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

Which has more kinetic energy:• 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!

How much kinetic energy an object has depends on 2 things:• Its MASS (kg)

and• Its VELOCITY (m/s)!

KE = ½ * M * V2

The UNIT for Energy is JOULE (J)!

EXAMPLE• 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 kgV = 15 m/s

KE = 1/2mv2

Step 2: Substitute the given values into the formula

Step 3: Solve the problem (with units!)

KE = ½ x 3000kg x (15 m/s)2

KE = ½ x 3000kg x 225 = 337,500 J

Practice on Your Own1. 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?

2. A 6-kg alley cat is chasing a mouse at a speed of 4 m/s. How much KE does the cat have?

3. Raven kicks a 1.5-kg soccer ball giving it a velocity of 3 m/s. How much KE does the ball have?