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Work, Energy, Kinetic Energy
and the Conservation of Energy
Work
▪ Work is the product of the force applied to an
object and the distance the object moves in the
direction of the force
Energy
▪ Energy is defined as the ability of an object to do
work.
▪ Therefore, if an object has energy, it can apply a
force and move an object through a distance.
Different Types of Energies
▪ An object can have an energy due to its position.
This is a potential energy.
▪ There are different types of potential energy. Some
examples are gravitational potential energy (the
energy stored in the gravitational field), elastic
potential energy (the energy stored in an elastic
object), and electrical potential energy (the energy
stored in an electric field).
▪ Potential energy is often thought of as a stored
energy.
More on Different Types of Energy
▪ Another type of energy is the energy due to
movement.
▪ This is known as Kinetic Energy.
▪ An object that is moving has the ability to do work
due to the fact that the object has a mass and is
moving (has a speed).
Kinetic Energy
▪ We are going to look at the amount of work that an
object can do because it has a mass and a speed.
▪ We will call the mass m and the speed v.
▪ We will have it moving across a frictionless,
horizontal surface when it runs into an object an
applies a constant force of F to the object.
Calculating the Amount of Work
W=F´ d
but
2
2
2 ´ a ´ d = v - v
or
2
2
v - v
d=
2a
The Kinetic Energy Formula
Therefore
2
2
v - v
W = m´ a ´
2a
or
2
m´ v
W=
2
Kinetic Energy
▪ The amount of work that the object can do because
it is moving is the Kinetic Energy or Ek.
▪ Energy is measured in Joules, just like Work.
Elastic Collisions
▪ The Law of Conservation of Energy states that
energy cannot be created or destroyed, it can only be
converted from one form to another.
▪ Unfortunately, some energies are difficult to
measure or contain. An example of this is the
conversion of kinetic energy into heat.
▪ Potential Energies and Kinetic Energy are much
easier to measure. These are known as the
Mechanical Energies.
Elastic Collisions
▪ If a collision conserves the mechanical energy, it is
called an elastic collision.
▪ For an elastic collision, we are able to use the
conservation of energy.
=
Etotal E total
Gravitational Potential Energy
▪ The gravtiational potential energy is the energy of
an object due to its position in the gravitational field
of the earth.
▪ The formula for the gravitational potential energy
near the surface of the earth is:
E =m g h
Elastic Potential Energy
▪ The elastic potential energy is the energy due to the
position of an elastic object.
▪ The elastic energy is directly proportional to the
change in the length of the elastic object.
▪ The formula for the elastic energy is:
k Dx
Ee =
2
2
Total Energy
▪ We can now keep track of three types of energies in
situations where the mechanical energy is conserved.
mv
k Dx
m v
k Dx 
+m g h+
=
+ m g h +
2
2
2
2
2
2
2
2