Kinetic and Potential EnergyUCR MESA

What is Energy?

What does it mean if you have a lot of energy?

For students it means you can run around, lift weights and scream.

If you have energy you can do things.

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What is Energy?

Energy is the ability to do work

This is similar to our every day definition of energy. You can’t run up the stairs (work) without energy!

We have lots of types of energy. Can you name a few?

Energy comes in many different forms

• Kinetic Energy

• Gravitational potential energy• Thermal Energy (heat)

• Nuclear Energy (nuclear power, atomic bomb)

• Electrical Energy (electricity)

• Chemical Energy (food, fire, …)

We are going to look at the first two types of energy

Kinetic Energy

Kinetic energy is the energy of motion

KE = ½ mv^2Units: Joules (J)

m = mass

v = velocity

Let’s Practice

What is the kinetic energy of this car when it is traveling 38.0 m/s?

What other information do you need?

KE = ½ m v^2

= ½ (1550)(38.0)^2

= 1119100 J

= 1.11 x 10^6 J

Car’s mass = 1550kg

Potential Energy

Gravitational Potential Energy is energy due to an objects height above the ground

PE = mgh

m = mass

g = acceleration due to gravity

h = height

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Let’s Practice A rock has a mass of 8.40 x 10^4

kg. The center of mass is 29.0 m above the ground. How much energy does it have?

PE = mghIdentify the variablesm= 8.40 x 10^4 kgg= 9.81 m/s^2h= 29.0 mPE = (8.4X10^4)(9.81)(29) = 2.39 x 10^7 J

http://gaaf.com/pictures/200406_utah/image009.htm

What’s so important about PE and KE?

We call the sum of PE and KE mechanical energy.

ME = KE + PE

Mechanical energy is important because it is conserved (as long as there are no non conservative forces, like friction)

Therefore, if one goes down, the other goes up by the same amount.

Conceptual understanding

A penny is dropped off the Eiffel tower (ignore air resistance). As it falls, what happens to it’s potential energy? What happens to it’s kinetic energy?

As it falls, its velocity goes up, so its kinetic energy goes up. It also looses height so its potential energy goes down.

However, mechanical energy stays the same ME = KE + PE

From the California Standards Test

Copyright © 2004 California Department of Education.

From the California Standards Test

Copyright © 2004 California Department of Education.

From the California Standards Test

Copyright © 2004 California Department of Education.