Physics Lesson11 Work and Energy Powerpoint

Key Points:
Lesson 11: Work and Energy
1. Understand the “Work Energy Theorem”, that work and
energy can be equal but that one cannot exist without the
other.
2. Understand Kinetic and Potential Energy.
3. Understand the relationship between force and work.

Energy, like motion, is a
measurable quantity.
Specifically, energy is a
measure of an object’s (or a
system’s) capacity to do work
(for example a brick held in your
hand has the potential to do
work on your toes if it is
dropped). Work is the transfer
of energy from one object (or
system) to another.
Work and Energy

Work and Energy
For instance, when you hit a tennis ball with your racket, we say the
ball has had work done on it. That is, it gained energy. In this case
the tennis racket caused the ball to move. The racket did work on
the ball, thus transferring energy to the ball.

Work and Energy
Work is also related to the force acting on an object. The
greater the force, the more work is done (on objects of the
same mass). Mass also effects the amount of work done as
a more massive object requires more energy to move.

Work and Energy
For a force to transfer energy to an object, the force must make
the object move (as in the diagram below).

Work and Energy
Motion does not just mean the movement of an object from one
place to another. When an object is compressed or stretched, the
atoms within it are moved. Thus changes in the shape of an object
constitute an exertion of force and thus result in work and energy
transfers.

Work and Energy
Work and Heat are transfers of energy. When the molecules of two
objects originally at different temperatures come into contact with
each other, they collide and transfer thermal energy. This transfer
of thermal energy is what we call heat, therefore heat is a transfer
of energy.
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Potential Energy
Potential energy is energy due to position or condition. Thus a rock
perched on a cliff in a Road Runner Wiley Coyote cartoon has
potential energy as does a stretched rubber band.

Potential Energy
Gravitational Potential Energy (GE p) is the potential energy of an
object due to its position (as in the diagram above). All forms of
potential energy must be described with respect to a chosen
reference point or condition.

Potential Energy
We can calculate GE p of an object using the equation below.
E p = mgh
Where:
E p is the gravitational potential energy of the object (in joules)
m is the object’s mass in kilograms
g is the acceleration due to gravity (on earth 9.81m/s 2 )
h is the object’s height above the ground (in meters)

Example 1:
A 3.00kg box is lifted by an upward force 1.50m above the surface
of earth to the top of a table. What is the potential energy of the
box in its new position?
Ep = mgh
= 3.00kg x 9.81m/s2 x 1.50m
= 44.1 J
The gravitational potential energy of the box is
now 44.1 J.

Example 2:
A 55.0kg diver is standing on a diving platform and has a
gravitational potential energy of 5.40 x10 3 J. What is the vertical
height of the platform?
E p = mgh
h = E p / mg
= (5.40 x 10 3 J) / (55.0kg)(9.81m/s 2 )
= 10.0m
The vertical height is 10.0m.

Kinetic Energy is Energy Due to Motion
When the potential energy of an object is released, that object often
begins to move. Moving objects can do work. A falling hammer can
drive a nail into wood. Moving molecules in hot steam can turn
turbines. Moving molecules in a sound wave can make your
eardrum vibrate.

Kinetic Energy is Energy Due to Motion
Any moving object has kinetic energy. When a solid object moves,
all the molecules move in unison. The kinetic energy of such an
object is often called mechanical kinetic energy. Even when
molecules are not physically attached to each other, they can still
move together like air molecules in the wind.

Kinetic Energy is Energy Due to Motion
Thermal energy is the kinetic energy of individual molecules moving
in a random manner.
The kinetic energy of a moving object is determined by the mass and
speed of the object. The relationship between kinetic energy, mass
and speed is represented by the following equation
E k = ½ mv 2
Where:
E k is the energy of the object (in Joules)
m is the mass of the object (in kilograms)
v is the velocity of the object (in meters per second).
This formula will still produce the proper units for energy
(recall that a joule is actually a kgm 2 /s 2 ).

Questions:
1. Which has more kinetic energy, a semi truck travelling
30km/hr or a bicycle travelling 30km/hr? Why?
2. Water does not always freeze at 0�C it can be cooled to –20
or 30�C if conditions are right. Which has more total thermal
energy, liquid water at -10�C or ice at the same
temperature? Why?
3. You are heating a substance but its temperature is not
increasing what type of energy is being increased? Why?
4. You are cooling hot iron in cold water, what kind of energy is
being lost by the iron? Why?
5. The water in the above question evaporates when the iron
touches it, what type of energy is being increased in the
water? Why?
6. You apply a force of 1000N to a baseball, and it travels a
distance of 80m. You apply this same force to a rail car and
it does not move at all, have you done work in both cases?
Why or why not?