Download Work Potential Energy Kinetic Energy

Work, Potential Energy, Kinetic
Energy & Power
Whoa!!!
I do!!! 
I just don’t
haveNow
any energy

F
Theforce
force,gives
F, pushes
the box
for a
The
the object
energy,
short distance.
This causes the
it does work!!!!!
box to start moving!!!!!!
Energy is the ability to use a force and move something a distance. Energy is the
ability to do work.
Work
If a force, F, is used to move an
object a distance, d, then the force
does work on the object.
W = Fd
F
d
Units for Work





The units for force are Newtons (N) and the
units for distance are meters (m).
Since Work is equal to force times distance, we
could use a Newton-meter (N·m).
But the more common unit is the Joule (J)
1 J = 1 N·m
1 J is defined as the amount of work for 1- N of
force to move an object 1-m
The weight lifter in the video lifts 280-kg a distance of 0.75-m.
How much work does he do during one lift?
The force needed to lift the barbell is equal to its weight!!!!
F = w = mg = (280)(9.8) = 2744-N
d = 0.75-m
W = Fd = (2744)(0.75) =2058-J
If he lifts it twice, how much work would he do?
F = 2744-N
W = Fd = (2744)(1.5) =4116-J
d = 2(0.75-m) = 1.5-m
Watch Video
Work
 Work
is the product of force and
distance.
 W = Fd
 If work is done on an object its
energy is changed
Lifting a Ball
This ball has a mass of 2-kg.
What is its weight?
w = mg = (2)(9.8) = 19.6-N
What force must be used to lift this ball?
Its weight: F = w = 19.6-N
How much work will be done to lift
this ball to a height of 5-m?
5-m
F = 19.6-N W = Fd = (19.6)(5) = 98-J
d = 5-m
Since 98-J of work was done on the ball its
energy was increased by 98-J. This was added
to a form of energy called Potential Energy.
19.6-N
Potential Energy
Potential Energy (PE) is energy stored in
an object due to its position, and is stored
by doing work.
For our ball, its position is its height,
5-m, above the ground.
98-J of work was
done lifting this
ball.
This form of potential energy is technically
It now has 98-J of
called Gravitational Potential Energy
potential energy
5-m
stored in it.
Gravitational Potential Energy is found by
multiplying the mass (m) of the object by
It now has the
gravity (g) and height (h).
ability to do 98-J
of work.
PE = mgh
PE = (2)(9.8)(9) = 98-J
A pile driver is a machine that uses the conversion of potential
energy into work to drive beams and poles into the ground.
The block at the top of the pile
driver has a mass of 275-kg
and is 9-m above the pole it is
driving into the ground.
What is the potential energy of
the block?
m = 275-kg
h = 9-m
Pile Driver
PE = mgh = (275)(9.8)(9)
= 24,255-J
This means the pile driver (block) can do 24,255-J of
work on the pole to drive it into the ground!!!!!!
A 2-kg ball is rolled up a ramp until it reaches a height of 5-m.
What is the potential energy of the ball?
m = 2-kg
h = 5-m
PE = mgh = (2)(9.8)(5) = 98-J
The same as if we
lifted it straight up!!!!
5-m
A 2-kg ball is rolled up a ramp until it reaches a height of 5-m.
What is the potential energy of the ball?
m = 2-kg
h = 5-m
PE = mgh = (2)(9.8)(5) = 98-J
The same as if we
lifted it straight up!!!!
5-m
When you are calculating the
potential energy of an object, all
you need to know is the mass
and height of the object.
How the object gets to the
height doesn’t matter!!!!!
No matter what path the 2-kg
ball takes to get to a height of
5-m, it still has a potential
energy of 98-J!!!
5-m
Lifting a Ball



When you lift a ball to a
certain height you do
work on it.
This work (W) is equal
to the weight of the ball
(w=mg) times the
height (h).
The work done to lift
the ball is stored as
potential energy.
PE = W = mgh
h
W = Fd =wh = mgh
Lifting a Ball



The ball now has potential
energy stored in it.
This means the ball has the
potential to do work if you
drop it on something.
But before it can do that
work it must convert the
PE to another form of
energy called kinetic
energy – energy of motion
PE = mgh
h
Kinetic Energy
Energy of Motion
Kinetic Energy
Kinetic Energy (KE)
is energy of motion.
 Kinetic Energy
depends on the
moving object’s mass
(m) and the square of
its speed (v).

1
2
KE = mv
2
Kinetic Energy Example
Find the kinetic energy of a 5000-kg Asian elephant running
at 4-m/s.
m = 5000kg
v = 4 ms
1
1
2
KE = mv = (5000)( 4)2
2
2
= 2500(16) = 40,000J
How could the elephant increase its kinetic energy?
The elephant could increase its mass, increase its speed or
both.
But changing its speed would be better since KE
depends more on speed.---Speed is squared!!!!!!
Kinetic Energy Example
The kinetic energy of a 5000-kg Asian elephant running at 4-m/s
was calculated to be 40,000-J.
What would its kinetic energy become if its mass doubled?
1
KE  mv 2
2
2  m  2  KE  2  40,000  80,000  J
What would its kinetic energy become if its speed doubled?
1
KE  mv 2
2
2  v  22  KE  4  KE  4  40,000  160,000  J