Energy

1. What is Energy?• “Something” that enables an object to do

work.• When you have “Energy” you can do work

• A. The amount of energy that an object has is related to the amount of work the object can do.

• The amount of energy can be precisely measured

• B. SI unit is Joules (for energy

and work)

• Comes in various forms

2. How do we get energy to do work??

• By eating• A. When you are

sick and not feeling well, do you have a lot of energy to go shopping or play basketball?

• No• B. Why?• Because we are not

eating.

3. What are the 2 main forces of Energy?

• A. Potential • 1. Stored Energy• A. Has stored

ability to move matter

• Kinetic• 1. Energy of

Motion

4. Types of Potential Energy• 1. Gravitational• 2. Elastic• 3. Chemical• 4. Nuclear• 5. Magnetic• 6. Electrostatic

5. What is Gravitational Potential Energy?

• Energy due to gravity acting on an object

PE = mgh m = mass in kg

g = acceleration due to gravity (m/s2)

h = height in meters

Gravity = force of attraction between any 2 objects)

6. What is Elastic Potential Energy?

• Elasticity• A. When stretching a rubber

band• The potential energy is the

energy that powers the rapid return to shorter original length.

• B. Ball bouncing• Round shape flattens as hits

ground – alters tension• Then returns to round shape

and allows it to bounce

7. What is chemical potential energy?• Energy stored within

the bonds of a substance.

• Uses Conservation of Mass – total amount of matter does not change

• Example:• A. eat cheeseburger• Gives energy to run• B. gas in a car

allows it to move.

8. What is nuclear potential energy?

• Energy stored in the bonds of subatomic particles

• Either split (fission)or fuse (fusion) of atoms

• Some mass is transformed into energy.

9. What is magnetic potential energy?

• Attraction or repulsion from magnetic energy stored in objects

• CME’s – coronal mass emissions from sun’s magnetic field

• Magnets – anything with iron in it

10. What is electrostatic potential energy?

• Attraction or repulsion of electrical charges

• Example: clean sock sticking to clothes after it comes out of a dryer

• Hair standing on

end

•  

11. Types of Kinetic Energy

• 1. Mechanical• 2. Thermal• 3. Electrical• 4. Nuclear• 5. Sound• 6. Electromagnetic• KE = 1/2mv2

• M = mass in kilograms• V = velocity (m/s)

12. Mechanical Vs Non-Mechanical Energy

Mechanical Energy• Energy that an object

has due to its motion or position

• Mass is, or will be, displaced (moved)

• Kinetic or Potential

• Non-Mechanical• Mass is not displaced,

no movement or velocity gained or lost

• Examples • Heat energy (fire)

• Chemical Energy• Gasoline• Food

13. What is thermal energy?

• Total energy in a substance measured in units of heat or temperature.

• Examples:• A. Ice melting• B. Water boiling• C. Warm water in a

shower• D. Frying an egg

14. What is electrical energy?

• Energy associated with movement of charges

• Examples:• A lit light bulb• Ex. Lightning – electricity• Positive and negative

charges collect in different parts of clouds

• When there are a lot of charges – get lightning

• Electricity travels between areas of opposite charges

15. What is sound energy?

• Energy transmitted through compression of matter

• Examples:• A. Hearing music • B. Hearing thunder

16. What is electromagnetic energy?

• Energy that travels in waves and can travel in a vacuum

• A. Visible light• B. Rainbow• C. Radio waves

Sample Problem #1

What is the potential energy of a 6kg bowling ball at a height of 20 meters?

Gravitational Potential energy is PE = (mass) x (Gravity) x ( Height )

= (6 kg)x (9.8 m/s2 ) x ( 20 m )= 1176 Joules

What is potential energy at the bottom (zero height)?

At a height of zero meters PE is zero Joules.

20 m

6 kg

Kinetic Energy (KE)• A stationary object has

zero kinetic energy

KE = ½mv2 KE= ½ x Mass x Speed2

Notice that speed is squared, so a small increase in speed will result in a larger increase in KE!

• Energy associated with an object due to its motion.

• If something is in motion it has the ability to do work

Sample Problem #2What is the kinetic energy of a 6kg

bowling ball, falling from a height of 20 meters just as it reaches the ground with a speed of 19.8 m/s?

Kinetic energy is

(Kinetic Energy) = ½ ( Mass ) x ( Speed )2

= ½ x( 6 kg ) x ( 19.8 m/s )2

= 1176.1 Joules

20 m

6 kg

20 m/s

17. Conservation of Energy

• Law of Conservation of Energy states that energy can never be created or destroyed. It can only be transformed from one form into another.• Example

• A book falls from a table• A rubber band pulled back

• The total Mechanical Energy must always be equal to the sum of Kinetic Energy and Potential Energy

Etotal = KE + PE

Pendulum“Like a child on a swing”

Energy exchange from PE to KE and back.

MaximumHeight

MaximumHeight, again

MaximumSpeed

Can predict speed from heightsince PE+KE constant

Check it out!

More than Meets the Eye

Watch below as the KE and the PE change as the car changes height and speed. Notice that no matter when you look at the two bars, there is only the same amount of energy that you started with.

Watch This!

18. Check for Knowledge

• Suppose you have a 1 kg mass at a height of 1.25 m. How much potential energy does it have?

• The mass is now let go. At the very bottom, what is it’s kinetic energy?

• What is the speed of the mass just before it hits the ground?