Download Work & Energy

Work, Power & Energy
How do they relate?
(Stone, Ebener, Watkins)
Work
Work – In science, work is defined as
force times distance.
Joule – The unit of work.
Work
• Work is the product of the force on an object
and the distance through which the object is
moved.
• Work = Force  Distance
•
W=Fxd
• So… F = ?
d=?
• The unit for work is the Newton-meter (N·m)
which is also called a Joule (J).

F
x
W =Fd
• Work is measured in
joules (J). 1 joule is equal
to a force of 1 N exerted
over a distance of 1 m.
How can I tell if work was done?
• A) Was the object moved by a force AND
a distance?
• B) Is there work done by another force
• C) Are you forcing something top move
against the influence of an opposing
force? (gravity or friction)
• D) Are you changing speed?
• E) Are you transferring energy to another
object?
Work or No Work
…………………….
A BOULDER FALLS OFF A
CLIFF. IT DROPS 2 M. IT HAS
A MASS OF 2 KG.
M= 10kg
In this case, the weight
does (positive) work
d=2m
Work = Mgd = (100N)(2m)
M= 10kg
Mg = 100N
Work = 200 Nm =200J
Practice Makes Perfect
Questions:
a)How much work is done when a weight
lifter lifts a barbell weighing 1000 Newtons
1.5 meters above the ground?
b)How much work is done when a weight
lifter pushes on a stationary wall with a
force of 1000 Newtons for 15 seconds?
Power
Power – The rate at which work is done
is called power.
Watt – The unit of power. It is equal to one joule
of work per second.
Power Problems
1) It takes 2 s for a car to drive down the street. If it
takes 1000 J of work for the engine to move the
car, how much power is used?
2) A women pushes a box across the floor with a
force of 600 N in 3 s. The displacement is 30 m.
How much power does she use?
Mechanical Energy
• When work is done on an object, the
object generally has acquired the
ability to do work.
• This "ability to do work" is called
energy and it has the same units as
work….Joules.
• Two Types of Mechanical Energy
– Potential Energy and Kinetic Energy
Potential Energy
• The energy that is stored is called
potential energy.
• An object’s ability to do work by virtue
of its position.
I’m learning
through
osmosis!
Potential Energy
Potential energy comes from the
position of an object relative to
the Earth. It is stored
energy. Potential energy is
not “used” until the object is
moved to a lower position.
Examples to Practice:
1. A 10 lb weight is lifted 5 ft. A 20 lb
weight is lifted 2.5 ft. Which lifting required
the most work?
(a) 10 lb weight
(b) 20 lb weight
(c) same work for each lifting
(d) not enough information is given to work
the problem
Examples to Practice:
2. A 50 kg kid rolls down a snow covered
hill that is 25 m high. What’s the kid’s speed
at the bottom of the hill?
(a) 22 m/s
(b) 22 m
(c) 1250 m/s
(d) not enough information is given to work
the problem
Examples to Practice:
3. A 4 kg book is sitting on a 2.4 m high
shelf.
A) How much PE does sit have?
B) How fast would it be traveling the
moment before it landed if it fell off the
shelf?
3 Types of Potential Energy (PE)
• Elastic PE
• Chemical PE
• Gravitational PE
• What are they? Examples of each? How
are they converted? To what?
Gravitational Potential
Energy
• PE = Weight  height
• PE = m g h
• Question:
– How much potential energy does a 10kg
mass have relative to the ground if it is 5
meter above the ground?
Roller Coaster energy
• Rollar Coasters
Kinetic Energy
The energy of motion. An object in motion
stores energy in this motion. It is the energy
of motion. It increases with speed. It also
can increase with mass.
Kinetic Energy
• KE is dependent upon velocity:
• Proportional to the square of your velocity :
if velocity doubles what happens to KE?
(quadruples)
• KE is also dependent on mass
• If mass doubles what happens to KE?
(doubles too)
Energy Conservation
• Energy is the
ability to do work.
It is the ability to
do work so it is
measured in
joules as well.
Law of Conservation of energy
• Energy can never be created or destroyed, just
transformed from one form into another. The sum of
the potential and kinetic energy will always be the
same, although some of the energy may be converted
to frictional heat, but it is not destroyed.
Law of Conservation of Energy
• Energy can change forms but
cannot be created nor destroyed.
• In a closed, isolated system, the
total ME (KE + PE) remains
constant.
• Examples??
PE = 1000J
KE = 0J
PE = 800J
KE = 200J
PE = 400J
KE = 600J
PE = 0J
KE = 1000J
PE = 500J
KE = 0J
PE = 100J
KE = 400J
PE = 0J
KE = 500J
PE = 100J
KE = 400J
Energy lost due to friction is actually not a loss; it is just a conversion.
PE (initial) = KE (final)
Conservation of Energy
The total energy of a system can be calculated by
using the Potential energy equation for the
object at it’s highest position.
EP = mgh = 10kg (9.8 m/s2) 30m
m = 10 kg
m = 10 kg
g = 9.8 m/s2
h = 30 m
30 m
EP= 2940J
This is the total energy that
the ball will ever have.
What about if the ball fell
one step? What would
happen to the energy?
Can you find the potential
energy of the ball on this step?
EP = mgh
EP= 10kg (9.8 m/s2) 20m What was the
m = 10 kg
g = 9.8 m/s2
EP= 1960J
h = 20 m
total energy of
this system?
2940 J
So what happened to the rest of the
energy? It was expressed as kinetic energy as
m = 10 kg
the ball fell.
How much kinetic energy?
Conservation of energy states the total
energy of the system should equal all the
parts of the energy. ETotal= EP + EK
30 m
20 m
2940 J = 1960 J + EK
EK = 980 J
Work/Energy Relationship
• If you want to move something, you
have to do work.
• The work done is equal to the change
in kinetic energy.
• Work = DKE
• Work is a transfer of Energy!
Work & Energy
Example Problem
1. A 100 kg mass is dropped from rest from a
height of 1 meter.
2. How much potential energy does it have when
it is released?
3. How much kinetic energy does it have just
before it hits the ground?
4. What is its speed just before impact?
5. How much work could it do if it were to strike a
nail before hitting the ground?
100 kg
KE = 12 mv 2 = 0
PE = mgh = (100kg)(9.8m / s 2 )(1m) = 980J
1 meter
100 kg
nail
100 kg
KE = 12 mv 2 = 980 Joules
PE = mgh = 0 Joules
Work Done = Force  Distace  980 Joules
Calculating KE &
Stopping Times
• http://www.wikihow.com/Calculate-KineticEnergy
• https://youtu.be/4yWA2Rk8wJM