Energy

Energy• “Something” that enables an object to do

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

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

• SI unit is Joules (J) – (N*m)

• Comes in various forms like ……..

• Chemical• bonds atoms and ions together.

• Electromagnetic - Electrical• moving electric charges.

• Electromagnetic - Light• Energy in the form of light

• Mechanical• associated with motion.

• Heat/Thermal• internal motion of particles.

• Nuclear • in the atomic nucleus.

• Nuclear fission & Nuclear fusion

Five main forms of energy:

Mechanical Energy• Energy due to the

position, or movement, of something

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

• Kinetic or Potential

• Non-Mechanical• Mass is not displaced,

no movement or velocity gained or lost

• Examples • Chemical Energy

• Gasoline• Food

Potential Energy

• Potential Energy• Energy an object has

due to position• Stored Energy

• Stored Work

• Examples • Why do we not walk

under ladders• Scared of Rubber

Bands

• Gravitational Potential Energy

• due to gravity acting on an object,

PE = mgh PE = mass x gravity x height

Metric unit of energy is Joules (same as for work).

Sample Problem

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 (10m/s/s ) x ( 20 m )= 1200 Joules

What is potential energy on the ground?At a h = 0 m, PE is zero Joules.

20 m

6 kg

Kinetic Energy (KE)• A stationary object

has zero kinetic energy

KE = ½ms2 KE= ½ x Mass x Speed2

Notice that speed is squared, If speed is doubled what happens to the KE?

• Energy associated with an object due to its motion.

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

• Examples• How do you stop

something that is moving

Sample ProblemWhat is the kinetic energy of a 6kg

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

v= 20m/s

Kinetic energy is

(Kinetic Energy) = ½ ( Mass ) x ( Speed ) x (Speed) = ½ x( 6 kg ) x ( 20 m/s ) x ( 20

m/s ) = 1200 Joules

20 m

6 kg

20 m/s

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• Draw out and label in Notebook

• 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!

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?