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ENERGY AND
WORK
Essential Question: How are energy, work and power
related?
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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
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–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
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• 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
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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
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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
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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!!!
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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
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Essential Question: What is the relationship
between potential energy and kinetic
energy?
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• 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
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• 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!
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• 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!
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The final energy is equal to the beginning
energy.
Law of Conservation of Energy Again!!!
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• 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!!!
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• 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
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• 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
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• 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
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• 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???
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• 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
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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!
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