EnergyIts lost, gained, found, stolen, etc. but does it even exist

Energy• “Something” that enables an object to do

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

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

• SI unit is Joules (J) – (N*m) – (kg*m2/s2)

• Comes in various forms like ……..

Five main forms of energy:

• Mechanical• associated with motion.

• Thermal (Heat)• internal motion of particles.

• Chemical• bonds atoms and ions together.

• Electromagnetic• moving electric charges.

• Nuclear • in the atomic nucleus.

• Nuclear fission & Nuclear fusion

Mechanical Energy• Energy due to the

position, or movement, of something

• Mass is, or will be, displaced

• 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 at zero height?At a height of zero meters PE is zero Joules.

20 m

6 kg

An automobile is transported 7.0 m above the dock. If its gravitational potential energy is 6.6 x 104 J, what’s its mass?

Kinetic Energy (KE)• A stationary object

has zero kinetic energy

KE = ½mv2 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 rolling at 20 m/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

ME = 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 for Knowledge

• One Arm Pete is playing with his son One Legged Leroy on the slide. The slide is 3.0 m high. It is (of course) frictionless. What is Little Leroy’s speed, at the bottom of the slide if he has a mass of 40 kg?

• What would One Arm Pete’s speed be at the bottom if he has a mass of 100 kg?

• Would their speeds be different if they fell off the back of the slide?

Energy Conversions• When a match is struck, three of the five types

of energy are on display. On a blank sheet of paper, write at least five lines (double spaced) discussing.• Ex. Before the match is struck, there exists

gravitational potential energy, heat energy, and chemical potential. Then, mechanical kinetic energy is used to convert the potential energy into heat energy.

Try this one…• A ball is thrown straight up in the air from the

ground. Jot down a quick picture of it and label at least one point where it has only kinetic energy, only gravitational potential energy, and both.