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Work and Energy
Work = Fd cos 
Where



F is the force
d is the displacement
 is the angle between the
force and the direction
F

d
Work = Fd cos 
 = ZERO in this situation!!!
F
d
Work
Net work in moving an object is
independent of the path to get there.
Same amount of work done
More force needed
Shorter distance traveled
Less force needed
Greater distance traveled
Energy – What is it?
The ability to do work
Types of Energy
Kinetic
Gravitational Potential
Elastic Potential
Heat
Light
Sound
Electrical
Chemical
Nuclear
Mechanical
Energy – energy
associated with an
object’s position or
motion
Energy - Kinetic
Kinetic Energy


This is the energy that an object possesses
due to its motion
If the mass of a body is m and its speed is
v then its kinetic energy is given by
KE = ½ m v2
Work-Energy Theorem
W = ΔKE = ½
2
mvf
–½
2
mvi
The net work done on an object
(by a net force) is equal to a
change in kinetic energy of the
object
Example problem
-A 10.0 kg sled is initially moving across a
frozen pond at a speed of 2.2 m/s. How
far will it travel if the coefficient of
friction between the sled and ice is 0.10?
Energy - Potential
• Potential Energy – the energy of an
object due to its position, shape, or
condition
• An system acquires potential energy
when work is done against another
force
Energy - Potential
Gravitational Potential Energy

This is the energy of an object associated
with its position in a gravitational field
(work done against the force of gravity to
put it there)
GPE = mgh



m = mass of the object
h = height of object above some fixed
position (the position is arbitrary)
g = the acceleration due to gravity
Energy - Potential
Elastic Potential Energy




This is the energy that an object possesses
due to its position of being stretched or
deformed (work done against the elastic
restoring force)
FOR A SPRING (or similar)…
EPE= ½ k x2
x = amount of stretch
k = the spring constant (a characteristic of
the object being stretched)
The Principle of Conservation
of Energy
Energy can be transformed from
one form to another, but it cannot
be created nor destroyed, i.e. the
total energy of a system is constant
Energy is measured in joules and it
is a scalar quantity