ENERGY AND

WORK

Essential Question: How are energy, work and power

related?

Types –– Kinetic – energy of motion

– Mechanical – done by machines

– Potential – stored energy (gravitational or elastic/flexed solids)

– Chemical – found in the bonds between atoms

Energy: The ability to do work

–Thermal – heat or internal kinetic energy

–Sound – vibrations in matter carried on waves

–Light – electromagnetic radiation

–Electrical – moving and static electrical charges

–Nuclear – found within the nucleus of atoms

• Work – a force exerted during a distance.

• W = F*d

• Work and Energy… You must have energy to do work.

• Unit is Joules (J)

Work

An acorn attempting to lift 230N (about 50 lbs.) one meter must pull with a force equal to the weight of the barbell.

How much work has he done?W = F*dW = 230N * 1mW = 230

Mechanical Work

230N

1 m

Unit?J Joules

230N

2.5m

• How much work to raise the bar over his head to the height of 2.5 m? *

• W= 230N*2.5m• W= 575J• If we doubled the weight to

460N, what happens to the amount of work?

*If his arms were long enough!

It would double as well! Directly

Proportional

Work and Vectors (direction)

• Work is a vector, which means it has a direction.

• Work can only be done if the force and distance directions are the same.

F

distance

NO WORK DONE!!!

Amount of work done per the amount of time it takes to do

it.

Power

Power =Work done

(J)Time (s)

Unit of power = J/s = 1 watt (W)1 kilowatt (kW) = 1000 W

Essential Question: What is the relationship

between potential energy and kinetic

energy?

• 1st Law of Thermodynamics:–Energy can be changed from one

form to another, but NOT destroyed.

• Change of energy: the final energy is equal to the beginning energy.

• The ultimate source of ALL energy on Earth IS?

Law of Conservation of Energy

• Gravitational–PE g = mass * gravity * height

–Higher an object = ____PE–Greater the mass = ____PE–Greater gravity = ____PE

• Elastic–PE elastic= weight x distance

stretched– Greater the weight = ____PE–Greater the distance stretched =

____PE

Potential Energy: Stored!

• KE = ½ mass * velocity2

–The more massive the object = ____KE

–The faster the object = ____KE

–Why does velocity have a greater role in KE than mass?

Kinetic Energy: Motion!

The mass is cut in half but the velocity is squared!

The final energy is equal to the beginning

energy.

Law of Conservation of Energy Again!!!

• KEi + PEi = KEf + PEf

• In other words, the sum total of KE +PE before and after some kind of transformation MUST be equal.

Law of Conservation of Energy – YET AGAIN!!!

• Entropy – a change of a useable form of energy into an unusable form… HEAT (thermal energy)

• Whenever energy is “lost” it is ALWAYS converted into HEAT.

2nd Law of Thermodynamics - Entropy

• Distance has to be in ______.• Force has to be in _______.

–How do you change kg to N?

• Time has to be in ______.• Mass has to be in ________.• Velocity has to be in _____.• How do you change W to kW?

Conversions

seconds

Newtons

x10

meters

kilograms

m/s

/103

• The bow stores potential energy in its bent limbs when drawn.

• This energy is transformed into kinetic energy as the arrow is released.

• PE of the bow = KE of arrow

• PE & KE = work the archer had to use to draw the bow

Conservation of Energy: bow and arrow

• Even though the arrow is now at rest (no KE) and the bow is not drawn (no PE), NO ENERGY HAS BEEN DESTROYED!

• The target did “Work” to the arrow by bringing it to rest with friction…

• Whenever energy is “lost” it is ALWAYS converted to HEAT.

Where does the energy go???

• What would happen to the depth of penetration if the arrow were moving faster?

• What happens to the speed of the arrow when shot from a “stiffer” (harder to draw) bow?

• What happens to the speed of the arrow if it weighs more?

• What happens to the speed of the arrow if it is longer? (bow pushes it for a longer distance)

Other thoughts

Equation Triangles

Fwd P

wt mg

PE

h ½m

KE

v2

Work (J) Power (W) PE (J) KE (J)

Work and energy have the same unit!