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WORK & ENERGYAP Physics C: Mechanics
What is Energy?
What is its definition?
What are its properties?
What forms can it be in?
The ability of a physical system to cause change
Energy can come in many forms and is transferrable. It is a
conserved quantity in the universe.
Chemical, electrical, nuclear,
thermodynamic, light, mechanical, kinetic,
potential, elastic, etc.
What is Work?What is its definition?
What are its properties?
How can it be calculated?
The amount of energy transferred by a force
acting through a displacement
Work (Energy) is a scalar quantity.
W=fscosθ
Work
W Fscos
W F s
The area under an Force-displacement graph
W Fx dx
WorkWork is an energy transfer.
If energy is added to the system then W is positive.
If energy is transferred from the system then W is negative.
WorkWork is a scalar.
The NET work on an object is the sum of the work done by each force acting on it.
WorkUnits of work
System Unit Alternate name
SI Nm Joule (J)
cgs dyne*cm erg
English foot*pound -
1 erg=10-7J
Work-Energy Theorem
W 1
2mv2 KE
W 1
2mvf
2 1
2mvi
2
The work done by a net force in displacing a particle equals the
change in kinetic energy of the particle. Stress on the word particle. This is
a drastic idealization.
Mechanical Potential Energy
Gravitational
Elastic
E g mgh
E las 1
2kx2
Conservation of Energy
This is always true, but mechanical energy is only conserved when only conservative forces are present.
Work done by a conservative force is path independent
Forces such as friction and air drag are non-conservative. They result in energy dissipated as heat.
What is it?
PowerThe time rate at which work is being done.
P W
t
How do we calculate it?
If a net force acts on a particle and does work W over an interval of time Δt, then the average power is:
Power The time rate of energy transfer.
P dW
dtInstantaneous Power
W F ds
dW F ds
dW
dtF
ds
dt
P F v
Power Watt are the units?
Joule
Second
Kgm2
s3
Watt
1hp=550ft*lbs/sec=746W
Note: A kilowatt-hour is often used by power companies. What is it a measure of?
Potential Energy and Conservative Forces
Potential energy is only associated with conservative forces. If an object or field does work on some external object, energy is transferred from
the object or field to the external object:
The energy transferred as work decreases the potential energy of the system from which it came.
Fx dx U
Potential Energy and Conservative Forces
The conservative force equals the negative (partial) derivative of the potential energy with respect to x.
The negative sign means that the final potential energy of the system is lower than the initial as the
conservative force does work.
Fx dU
dx
Potential Energy and Conservative Forces
Examples:
Fs dU
dx
d1
2kx2
dx kx
Fg dU
dy
dmgy
dy mg
E mc2