energy, work & power
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Regents Physics Mr. Rockensies. Energy, Work & Power. Introduction to Energy. How does energy manifest itself physically?. What is energy?. Energy is the ability to do Work . Energy exists in several different forms. Almost all energy on Earth can be traced back to the Sun. Examples: - PowerPoint PPT PresentationTRANSCRIPT
Energy, Work & Power
Regents PhysicsMr. Rockensies
Introduction to EnergyHow does energy manifest itself physically?
What is energy?
Energy is the ability to do Work.
Energy exists in several different forms. Almost all energy on Earth can be traced back to the Sun.
Examples:Mechanical, Thermal (Heat), Chemical, Nuclear, Electromagnetic, Kinetic, Potential
What is Work?
Work is a measure of the amount of energy used to move an object.
Mathematically: Work = Force • displacementBecause we are multiplying two vectors, we end up with a scalar quantity – Work is a scalar quantity.
Work is only done when the Force and the displacement are in the SAME direction.
Unit for work:
Work = F • d = Newton•m
To simplify this, we use Joules (J).
1 J = 1 N•m
All energy quantities will be measured in Joules.
Work and Friction
When friction is present, energy in a system is lost because of it.When two objects come into contact and move past each other, energy is given off in the form of HEAT – this is due to FRICTION.
What is Power?
Power is the rate at which energy is used, transformed, or transferred.
Power = Work/time AND
Power = Velocity • Force Power = Force • Distance
timeUnits: Watts = Joules/second = N •m = kg • m2
s s3
We measure Power in Watts to simplify the units
What is horsepower then?
A measure of the amount of energy expended by the average horse over a given period of time. One horsepower is equal to 746 Watts, but depending on how you are using horsepower, it is up for debate.
Practice Problems
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Law of Conservation of Energy
Energy cannot be created or destroyed; it can only change forms.
Conservation in Free Fall
Ei = Ef
PEi = PEf + Kef
mghi = mghf + 1/2mvf
2
ghi – ghf = v2/2Δhi
Δhf
m
m
initial
final
vf
If an object starts from some initial height and enters free fall, its initial Potential Energy will equal the sum of its final Potential Energy and Kinetic Energy
Energy is a scalar quantityBecause energy is a scalar , direction doesn’t matter, and this works for curved motion as well
hi
m
m
hf
vf
Hill slides and roller coasters are the same
A
B
C
EA = EB = EC
PEA + KEA = PEB + KEB = …
hA
hC
Pendulum
vi = 0Kei = 0All PE
vf is maxKE is maxNo PE
Etop = Ebottom
PE = KE
Springs
Ei = Ef
Pes = Kecart
1/2kx2 = 1/2mv2
PE = 1/2kx2
KE = 1/2mv2
Systems with Friction
Initial Mechanical energy (PE + KE) becomes final mechanical energy plus some internal energy, converted by friction. (PE + KE + Q)
height reached with no friction
height reached with friction
Q = internal energy, usually heat, caused by friction
Summary
In a closed isolated system, what happens to the gravitational potential energy of an object as it falls? the kinetic energy? the total energy?
Does the amount of energy an object has change if the object changes direction?
How does Potential Energy change if a spring is compressed more?
When is Kinetic Energy the greatest for a swinging pendulum?
Energy Conservation Practice
Energy Conservation Practice
Energy Conservation Practice
Energy Conservation Practice
Energy Conservation Practice
Energy Conservation Practice