Transcript What is energy?

Chapter 12: Work & Energy
Section 1 – Work & Power
Imagine that you need to change a flat tire.
The car has to be lifted off the ground.
If you tried to do this yourself, you could use a great
amount of force and the car would not budge!
Did you do any work while trying to lift
the car?
NO, you didn’t!
In order to understand why no work was
done, lets look at the definition of
WORK.
• Definition: work – the use of force to
cause an object to accelerate in the
direction of the force.
Work = force x distance (W=F * d)
Work is the amount of energy needed to move
an object.
Work is only done when a force causes an
object to accelerate (move).
Even though you put a lot of force into the
car, it did not move.
•So, the work done on the car was 0.
• Work is measured in units called Joules (J).
To put it in perspective:
You do about 1 J of work when you
lift an apple from your waist to the
top of your head.
That’s about 1 meter.
Work?
• Lets look at an example…
Raymond pushes a crate 10 meters. He
used 60 Newtons of force to move the
crate.
How much work did he do?
W = (60 N) x (10 m) = 600 J
Raymond did 600 Joules of work.
Work?
• Lets look at an example…
Ed exerts 250 Joules of work to lift a box 1
meter off the ground.
How much did the box weigh?
•Remember that weight is a force!
W = F x d ; so, F = W/d
F = 250 J / 1 m = 250 N
The box weighed 250 N ( 56 lbs )
What if we wanted to measure how much work
is done over a certain period of time?
Then, we would measure the POWER used!
• Definition: power – the rate at which
work is done over a period of time.
P = W/t
•Power is measured in Watts (W)
The quicker work is done, the more power
it takes.
Work done at a slower speed takes less
power.
Example: You do 200 J of work.
If you do it over 2
seconds, you would use
100 W of power.
If you do it over 10
seconds, you would
use 20 W of power.
P = 200 J / 2 s = 100 W
P = 200J / 10 s = 20 W
Power & Time Connections
Think about an electric mixer…
On a slow speed, the mixer does not move
very fast.
It will take a long time to mix something.
Power & Time Connections
If you want to decrease the amount of
time it takes to mix something…
Then you increase the power level.
The mixer does the same amount of work,
but in much less time!
Chapter 12: Work & Energy
Section 2 – Machines
Machines
• Machines make work easier to do.
They do not decrease the amount of
work done!
Machines are all around us!
• Definition: simple machine – one of the six
basic machines.
All complex machines are made from
simple machines joined together.
Example: The wheel is a simple machine…
•A car is a complex machine made of wheels,
levers, and other types of simple machines.
The 6 Simple Machines: Lever
• Definition: lever – a machine composed of an
arm and fulcrum.
The fulcrum is the pivot point of the lever.
Levers are often used to lift objects.
Real Examples of Levers
The 6 Simple Machines: Pulleys
• Definition: pulley – a rotating wheel used to
lift or pull objects.
The 6 Simple Machines: Wheel & Axle
• Definition: wheel & axle – a simple machine
that consists of two circular objects of
different sizes.
The wheel is the larger object.
The 6 Simple Machines: Inclined Planes
• Definition: inclined plane – a flat surface with
endpoints at different heights.
There are three types of inclined planes:
Ramps – IP’s that make it easier
to lift objects to different heights.
The 6 Simple Machines: Inclined Planes
Definition: wedge – an IP that is forced
between two objects.
The 6 Simple Machines: SCREWS
• Definition: screw – an IP that is wrapped
around a cylinder.
Machines…
• Definition: compound machine – a machine
composed of 2 or more simple machines.
Machines
Some machines work better than others,
obviously…
Definition: mechanical advantage – the
advantage created by a machine that allows
work to be done easier.
Choose the right machine for the job
to get the biggest advantage!
Mechanical Advantage
• Tells how much a machine multipies force
or increases distance
• The ratio between the output force and the
input force
• Mechanical Advantage= output force
input force
or
Mechanical Advantage = input distance
output distance
Mechanical Advantage
• Machines with a MA greater than 1
multiplies the input force.
• Machines with a MA of less than 1 does
not multiply force, but increases distance
and speed.
Mechanical Advantage Problem
• Calculate the mechanical advantage of a
ramp that is 5.0 m long and 1.5 m high.
Given: input distance = 5.0 m
Output distance = 1.5 m
Unknown: mechanical advantage=?
MA= input force
output force
MA= 5.0 m / 1.5 m
MA = 3.3
Mechanical Advantage Problem
• A bus driver applies a force of 55.0 N to
the steering wheel, which in turn applies
132 N of force on the steering column.
What is the mechanical advantage of the
steering wheel?
• MA = output force/input force
• MA = 132 N / 55 N
• MA = 2.4
When using machines, some of the energy
we put into the machine is lost.
The more energy that a machine can keep…
•The more efficient that machine is.
Definition: efficiency – a measure of how
much useful work a machine can do.
•Friction often causes a machine to lose
efficiency.
Efficiency
• A measure of how much useful work a
machine can do
• The ratio of useful work output to total work
input
• Efficiency = useful work output
work input
• Usually expressed as a percentage
• To change an answer found using the
efficiency equation to a percentage, multiply
the answer by 100 and add the % sign.
Efficiency Problem
• A sailor uses a rope and an old, squeaky pulley to raise a
sail that weighs 140 N. He finds that he must do 180 J of
work on the rope in order to raise the sail by 1 m (doing
140 J of work on the sail). What is the efficiency of the
pulley? Express your answer as a percent.
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Given: work input = 180 J, useful work output = 140 J
Unknown: efficiency = ? %
Efficiency = useful work output/work input
Efficiency = 140 J / 180 J
Efficiency = 0.78
Efficiency = 0.78 x 100 = 78%
Efficiency Problem
• Alice and Jim calculate that they must do 1800 J of work
to push a piano up a ramp. However, because they must
also overcome friction, they actually must do 2400 J of
work. What is the efficiency of the ramp?
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Efficiency = useful work output/work input
Useful work output = 1800 J
Work input = 2400 J
Efficiency = 1800 J / 2400 J
Efficiency = 0.75
Efficiency = 0.75 x 100 = 75%
Efficiency Problem
• It takes 1200 J of work to lift the car high
enough to change a tire. How much work
must be done by the person operating the
jack if the jack is 25% efficient?
Efficiency = useful work output/ work input
0.25 = 1200/work input
Work input = 1200/0.25
Work input = 4800 J
Chapter 12: Work & Energy
Section 3 – What is energy?
Energy comes in many forms that we are
familiar with.
Light
Heat
Sound
• Definition: energy – a measure of the
ability to do work.
•Energy is measured in Joules (J)
Hey… Work has the same unit as energy!
Thermal & Light Energy
Energy
• Energy comes in many forms…
Definition: mechanical energy – energy that can
be used to do physical work.
• Examples:
 Sound Waves
 Objects in Motion (kinetic)
Definition: chemical energy – energy stored in the bonds
of atoms.
• Examples:
 Burning Gasoline
 Batteries
 Food
Energy
Definition: electrical energy – energy
resulting from the flow of electrons.
•Examples:
 Electricity
 Lightning
Definition: radiant energy – energy travelling as
electromagnetic waves.
•Examples:
 Sunlight
 Heat
Energy is very closely related to work. In fact…
• Energy must be transferred to do work!
It takes energy to do pretty
much anything.
Energy is constantly flowing
through the universe.
When you pull a rubber band back, you are
doing work on the rubber band.
By doing that work, you are transferring some
of your energy into the rubber band.
You used energy to do the
work that stretched the
rubber band!
Now, the rubber band has
the energy you used!
Mechanical Energy comes in 2
Great-Tasting Flavors!
Potential Energy
When you stretched the rubber band, the energy you transferred to it
was held as “potential energy”…
• Definition: potential energy – the stored
energy that results from an object’s
position or condition.
Potential Energy
When an object is stretched or
compressed, it has “elastic” potential
energy.
Potential Energy
When an object is above the ground, it
has “gravitational” potential energy.
We will focus on GPE.
Gravitational Potential Energy
GPE depends on mass and height of an
object.
The GPE equation:
•PE = mgh
 m = mass (kg)
 g = gravitational acceleration (9.8 m/s2)
 h = height (m)
Gravitational Potential Energy
Problem
• A 65 kg rock climber ascends a cliff. What is the
climber’s GPE at a point 35 m above the base of
the cliff?
 Given: mass (m) = 65 kg
Height (h) = 35 m
free-fall acceleration (g) = 9.8 m/s2
 Unknown: PE = ? J
 PE = mgh
 PE = (65 kg)(9.8 m/s2)(35 m)
 PE = 22,000 J
Gravitational Potential Energy
Problem
• Calculate the GPE of a car with a mass of 1200
kg at the top of a 42 m hill.
 Given: m = 1200 kg; h = 42 m; g = 9.8 m/s2
 Unknown: PE = ? J
 PE = mgh
 PE = (42)(9.8)(1200)
 PE = 490,000 J
GPE
at
Work
In the example with the rubber band…after you released the rubber
band…
It had kinetic energy as it snapped back into place.
• Definition: kinetic energy – the energy an
object has because of its motion.
Only MOVING objects have
kinetic energy!
What would happen to a bottle cap if the rubber
band hit it?
Kinetic Energy
OMG!!!
Kinetic Energy!
The kinetic energy equation:
KE = ½ mv2
m = mass (kg)
v = velocity (m/s)
Higher velocity gives increases your KE
more than a higher mass.
•This is because velocity is squared!
Kinetic Energy Problem
• What is the kinetic energy of a 44 kg
cheetah running at 31 m/s?
Given: mass (m) = 44 kg; speed (v) = 31 m/s
Unknown: KE = ? J
KE = ½ mv2
KE = ½ (44)(31)2
KE = (22)(961)
KE = 21,142 J (2.1 x 104 J)
Kinetic Energy Problem
• What is the kinetic energy in joules of a
1500 kg car moving at 18 m/s ?
Given: m = 1500 kg; v = 18 m/s
Unknown: KE = ? J
KE = ½ mv2
KE = ½ (1500)(18)2
KE = (750)(324)
KE = 243, 000 J (2.43 x 105 J)
Kinetic Energy Problem
• A 35 kg child has 190 J of kinetic energy after
sledding down a hill. What is the child’s speed in
meters per second at the bottom of the hill?
 Given: m = 35 kg; KE = 190 J
 Unknown: V = ? m/s
 KE = ½ mv2
 190 = ½ (35)v2
 380 = 35 v2
 V2 = 10.9
 V= 3.3 m/s
What happens to energy??
When you hit a baseball, what happens to the energy
that you transferred to the bat?
Did the energy disappear, or did it just change into
other forms…?
What happens to energy??
When you hit the baseball, the kinetic energy of the
swinging bat is transferred to the baseball.
The baseball flies away!
What happens to energy??
Energy is also used to produce the cracking
sound..
And some energy is used to heat up the bat and
the baseball!
What happens to energy??
• What happened with the baseball is an example of
a very important law…
The Law of Conservation
of Energy.
• The Law of Conservation of Energy states:
ENERGY CAN NEVER BE
CREATED OR
DESTROYED.
It is always
transferred.
Energy can change forms!
What point has the most PE?
What about the most KE?