Transcript Energy - Teacher Notes

ENERGY
Ability to do
WORK
Measured in
Joules (calories
or kilocalories)
What is Energy?

Work depends on:
◦ The amount of force applied to the object.
◦ The distance that the object moves while the
force is applied.
◦ The direction of the force with respect to the
direction the object moves.
What is Work?
Work:
Work is done on an object whenever a force is exerted on an
object through some distance.
If the force is constant and parallel to the direction of
movement then:
W=Fd
W = 0, if d = 0
No work done if object
doesn’t move.
W = 0, if F = 0
No work done, whenever
there is no force.

If the force on the object is in the
direction the object moves, the work done
is:
W = Fx
F
x

If the direction of the force is opposite
the direction the object moves, work is:
W = -Fx
F
x
If the force is perpendicular to the
direction the object moves, the work done
is 0.
 If the object doesn’t move, the work
done is 0.

F
W=0
x
Force is NOT Work
How is all energy divided?
All Energy
Potential
Energy
Gravitation
Potential
Energy
Elastic
Potential
Energy
Kinetic
Energy
Chemical
Potential
Energy
What is Potential Energy?
o Energy
that is stored
and waiting to be
used later
What is Gravitational Potential Energy?
Don’t look down,
Rover!
Good boy!
o
Potential energy due
to an object’s
position
o
P.E. (gravitational) =
mass x
height x gravity
What is Elastic Potential Energy?
o
Potential energy due compression or
expansion of an elastic object.
Notice the ball compressing
and expanding
Work done by a spring =
½ Kx2
What is Chemical Potential Energy?
o
Potential energy
stored within the
chemical bonds of an
object
An object’s kinetic energy depends on:
 the object’s mass.

◦ Kinetic energy is directly proportional to
mass.
the object’s speed.
o Kinetic energy is directly proportional
to the square of the object’s speed.
o Energy an object has due to its motion
K.E. = ½ mv2

What is Kinetic Energy?
Kinetic energy is a scalar quantity.
 When the kinetic energy of an object
changes, work has been done on the
object.
 Work is a scalar quantity.


What is the kinetic energy of an object
with mass of 1 kg, moving at 1 m/s?

0.5 Joule.

The work done on an object by the net
force equals the object’s change in
kinetic energy.
Wnet = DKE
Work and Kinetic Energy
Law of Conservation of Energy


If no external forces act on a
system, the total energy of the
system will remain constant.
Mechanical Energy =
PE + KE
What are different Forms of
Energy/Energy Transformations?
Suppose you have to exert a force of 10 Newtons to push a
book 1 meter across a horizontal table at constant velocity (as
shown above). Since your force is in the direction the book
moves, the work you do on the book is given by:
Your work = Fx = (10 N)(1 m) = 10 Joules
Work done by Friction
•The work-energy equation would lead you to think that doing 10
Joules of work on the book should increase the kinetic energy of the
book by 10 Joules - but that doesn't happen (the speed of the book is
constant). What's going on?
•A friction force opposes the motion of the book. This force must also
be 10 Newtons (Since the book moves at constant velocity, the net
force on it must be zero.).
•The friction force pulls in the opposite direction from the direction
the book moves, the work done by friction is given by:
Work done by friction = -Fx = -(10 N)(1 m) = -10 Joules
•This means that the friction force removes 10 Joules of energy from
the book. So, while you were adding 10 Joules of energy to the book,
friction was busy taking the 10 Joules of energy away from the book.
This is why the kinetic energy of the book does not change!
Power is the rate work is done.
 Units of power: 1 Joule/sec = 1 Watt
 1000 Watts = 1 kilowatt
 Power is a scalar quantity.

Power = DWork
time
W
P
t
A machine is a mechanical device used to
do work.
 Examples of simple machines:

◦ Inclined plane
◦ Lever
◦ pulley
What are (Simple) Machines?
A machine can never output more work
(energy) than is put into it.
 At best,

Workout = Workin
Workin
Machine
Workout
(Simple) Machines
Machines can’t multiply work or energy,
but they can multiply force.
 Mechanical advantage measures how
much a machine multiplies force.

MA =
Force machine exerts
Force you exert
What is Mechanical Advantage?
The efficiency of a machine tells how
much of the energy (work) that goes into
the machine actually does useful work.
 It is usually expressed as a percent.

Efficiency =
Useful work done
Energy input
x 100%
How do we Measure Efficiency?