Download Work - Mr Bernabo at Affton High School

Have students do lab:
using ramp measure speed at 10 cm height,
then predict height for double speed.
do 20, 30, 40, -- cm
and graph.
twice as fast = 4 times the energy
(can also do impression into clay with marbles
twice as fast = 4 times deep impresssion
Start bringing books daily (starting tomorrow).
We won’t use them everyday…. but pretty
frequently
Chapter 8
ENERGY
POWER
WORK
WORK
Has a different meaning in PHYSICS
WORK
W=Fd
work done on
an object
force applied
to the object
distance the
object moves
Work and impulse are similar but different
Impulse = F Dt
(time)
Work = F d
(distance)
Impulse = F Dt
Work = F d
this is important!!
= the change
in momentum
of an object
= the change
in ENERGY
of an object
In order to do work on an object a
FORCE has to push or pull an object
and MOVE IT
Work = F d
Force
Displacement
A person pushes a crate horizontally
across a floor with a force of 30 N for a
distance of 5 m.
How much work is done on the crate?
Work = F d
N
Nm
m
Also called a Joule (J)
Or
kg m2 If you prefer
s2
Two crates are pushed with equal forces.
One is pushed twice as far.
Compare the amount of work done.
1m
2m
Work = F d
Two crates are lifted equal distances,
one crate is twice as heavy.
20 kg
10
kg
Work = F d
A crate is twice as heavy and
is lifted twice as far.
20 kg
10
kg
Work = F d
An inmate pushes hard against a solid
wall but it doesn’t budge.
No work is done, why?
Work done on an object falls into 2
basic situations
1.) Pushing an object against another force
Lifting weights (at a constant speed)
Pushing a crate across a rough floor
(at a constant speed)
A person lifts a 650 N weight up 0.45 m.
How much work is done on the weight?
How would you answer change if the object
instead had a MASS of 650 kg?
A weight lifter holds 560 N mass over his head
for 10.0 seconds. How much work is done?
W=Fd
0
Work done on an object falls into 2
basic situations
2.) Pushing an object to accelerate it
Speeding up or slowing down a car
What is the force
doing work on the
apple and the car?
Work = Force x Distance
Lastly in order to count as “work”,
the force has to be in the parallel with the
displacement of the object
The force does work on the box
displacement
Work = Force x Distance
Does this force do work on the box?
Force
displacement
rce
Work = Force x Distance
Work is not done by the force
Only Part of this force will do work
Force
distance
How much work is done carrying a 200 N
crate across a level floor for 10 m at a
constant velocity?
How much work is done?
2 meter
1 meter
1 meter
1/2 meter
10 N
10 N
20 N
20 N
SHOW WORK WHERE APPLICABLE
do first side of work / power ws???
Move on
POWER
Again a little different in physics
Power:
is the rate at which work is done
What does rate mean here?
Speed is the rate at which distance changes
Acceleration is the rate at which velocity changes.
POWER =
Work done (J)
Time it took to do it (s)
W
P=
t
W
P=
t
Which is more powerful a motor which can do:
100 J of work in 2 seconds
or
100 J of work in 1 seconds
Work (J)
POWER =
Time (s)
POWER has units of J/s
Also called a watt (W)
Or
kg m2 If you prefer
s3
An electric device like a heater or light
bulb is rated in watts.
This tells you the rate at which it uses electrical
energy.
W
P=
t
What is the power of a
motor which can do 60 J
of work in 2 seconds?
60
J
P=
= 30 W
2s
W
P=
t
keep in mind
that the
variable W is
work
but the unit W
is Watt for
power
60
J
P=
= 30 W
2s
Running up a set of stairs
Walking up a set of stairs
Which requires more work?
Which requires more power?
Both motors do the same amount
of work but one can do it faster
because it has more power
1 kg
1 kg
What about here?
What’s the same & different?
1 kg
10 kg
What about here?
1 kg
1 kg
Force x distance
POWER =
Work
Time
What is the power requirement on a motor which
needs to to lift a 1100 kg car a distance of 3.0
meters in 12 seconds?
A motor which is 3
times a powerful could
do it in….
Other units of power
1 kW = 1000 W
1 hp = 746 W
SHOW WORK WHERE APPLICABLE
(* questions)
Work/Power WS
To review (optional)
read pages 103-105 in book
What is energy
How do you get it?
Where does it go?
What gives an object like you the ability
to do work?
ENERGY
When you do work on a sledgehammer by
lifting it up, it then HAS the ability to do
work on something else
What it has when it is lifted is…
ENERGY
ENERGY
is the ability to do work.
ENERGY has many forms
MECHANICAL ENERGY
(what we will focus on)
has 2 TYPES
Potential EnergyEnergy due to the POSITION of an object
Kinetic EnergyEnergy due to the MOTION of an object
Mucho importante concept
Work = DEnergy
The amount of work done on/by an object is equal to energy
gained or lost
An object has potential energy due to
its position.
This energy can be used to do work
on something.
How does the bow gain PE?
The work done pulling the string back is equal
to the PE stored in the bow.
Work = F d
Because a position is relative
( or compared to something else)
Potential Energy is always a comparison
(not absolute measurement)
10 J of PE
What is the amount of PE in the
stretched bow compared to?
10 J of PE
If the stretched bow has 10 J of energy
and the string was pulled back 0.50 m,
with what force was the string pulled.
Objects (like a bow) which go
back to their original shapes after
being deformed are said to be
ELASTIC.
The energy stored by changing the shape of
an elastic object is called
ELASTIC POTENTIAL ENERGY
Can you think of some elastic objects
which could store Elastic PE?
Remember when work is done on an
object, it gains that amount of energy.
Does it take work to LIFT an object?
It requires WORK to lift an up object up,
and it gains that energy as:
Gravitational Potential Energy
How much work is done to
lift a 30 kg rock up 2 meters?
How much PE does it gain?
Gravitational PE = weight x height
PE = m g h
h
Energy has the same units as work
Joules (J)
Or Nm
Or
kg m2 If you prefer
s2
A 5.0 kg rock is lifted 2.0 meters how much
PE does it gain?
PE = 5.0 kg * 9.8 m/s2* 2.0 m
2.0 m
Remember that position is relative. What are we
comparing the position of the elevated rock to?
PE = 98 J
2.0 m
Since it is 2 m above the ground,
its PE is 98 J more than if it were
on the ground.
But that’s not the whole picture
2.0 m
You can only compare the PE of an object
at 2 different locations
2.0 m
3.0 m
How much PE does the rock have
compared to the lower floor?
2.0 m
3.0 m
In which position does the rock have the most PE,
given that all are 3 m above the ground?
Neglecting friction the work done in each
scenario is the same and each gained the
same PE
Two magnets repel each other how would
you increase their potential energy?
What if one is switched so that they
both attract?
In all of our examples of work so far, the
Force that we applied (over a distance)
was balanced by another force on the
object.
such as
Gravity
Friction
Springy Things
We can still do work on an object even if we are not
working against another force acting on the object.
It just becomes a different type of energy
KINETIC ENERGY
Force
distance
What things would affect how much
energy a moving object has?
Its speed or velocity
&
Mass
velocity (m/s)
Mass (kg)
KE =
2
mv
2
Kinetic Energy = ½ mass x velocity2
Which has a bigger impact on KE,
Mass or velocity?
KE =
2
mv
2
If one object has twice the mass but
same speed its KE is
TWICE AS MUCH
KE =
2
mv
2
If one object has the same mass but
twice the speed its KE is
FOUR TIMES AS MUCH
KE =
2
mv
2
How much kinetic energy does a 2.0 kg object
If it is moving at 3 m/s?
If it is moving at 6 m/s?
KE =
2
mv
2
How much kinetic energy does a 23 kg
object at rest have?
Don’t confuse Kinetic Energy with momentum
KE =
2
mv
2
p = mv
How can a mouse and an elephant have the
same kinetic energy?
If a car has twice the KE of another equal mass
car, is it moving twice as fast?
Whatmuch
if youkinetic
(and the
ball) does
were aon
an
How
energy
ball
airplane
traveling
very
fast.
have sitting
in your
hand?
The ball would have no KE relative to the plane
but….
a lot relative to the ground
Because speed is relative so it KE!!
Some times when energy is added to a
substance it gets hotter (temperature goes up)
What type of energy does Temperature
indicate (KE or PE)
Atoms at different temperatures applet
Moving atoms have KINETIC ENERGY
The higher the Temperature
The greater their KE
The work done on an object is
equal to its change in KE
Work = DKE
How much KE will an object have if
it starts from rest and 150 J of work is
done on it?
A 100 N force is applied over 5 m to a:
1 kg car at rest
&
2 kg car at rest
Afterwards which car has more KE
Afterwards which car is moving faster?
Objects with kinetic energy can do work!
When they impact the wagon exerts
a force on the crate
force
distance
Objects with kinetic energy can do work!
force
distance
The more KE energy an object has the more
work it can do before coming to a stop
DKE = Work
2
mv
2
Kinetic Energy
of an object
=Fd
How much work
it can do before
stopping
Or the wagon can do work on a spring storing
it in the spring as elastic potential energy
distance
As the wagon does work it loses energy and
the spring gains it.
distance
IN GENERAL
Work is equal to the change in
energy of an object
Work = DE
A 10 kg car is moving at 5 m/s
What is the car’s KE
How much work would be done to stop it?
How far will it travel if the
stopping force is 10 N?
Compare the stopping distances for cars of
equal mass, but different speeds
(hint: think KE)
10 m/s
1m
20 m/s
4m
40 m/s
16 m
SHOW WORK on underlined
Book Problems 119-121
PE/KE 7, 9, 22, 24, 27, 31, 32, 33,34
I.) How high would a 5.0 kg object need
to be lifted to have 120 J of PE?
II.) How fast would a 5.0 kg object need
to be moving to have 120 J of KE?
III.)
How many Joules of elastic potential energy
is stored in a rubber band by stretching it a
distance of 10 cm by a force of 2.5 N?`
ENERGY is conserved
What does it mean in science when
something is conserved?
Uh Like,
quit wasting
electricity
NO
Energy is conserved means:
Initial Energy = Final Energy
Energy is conserved means:
The total amount of energy doesn’t
change. But the energy can change
form ( PE / KE / Heat)
Force x distance
Total Energy is Conserved
Picture a pendulum (on a string)
Where
Wheredoes
doesitithave
havethe
themost
mostPotential
Kinetic Energy?
Energy?
Picture a pendulum (on a string)
KE
PE
If itthe
starts
with
Jall
of
PE,
how
much
How
dosides,
we
know
that
at
sides,
it
At
At
The
the
total
bottom,
amount
theit10
energy
isof
energy
Kinetic
isboth
all
isPE.
constant
Energy
kinetic
energy
does
it have
at the
has the
amount
of PE?
itsame
changes
form
bottom?
My “Real” Pendulum behaves a little differently.
Does the energy appear to be constant?
Gravity
doesitwork
on
Just before
hits the
the
massitand
changes
ground
is all
in the
the
form
of the
form
of kinetic
energy
energy
Gravitational
PE
Heat
Kinetic
Energy
When it hits the
ground,
the mass does work
on the ground
Gravitational
PE
Heat
Kinetic
Energy
Even though the
energy changes we
always have the same
amount
Gravitational
PE
Heat
Kinetic
Energy
Changes in energy for a bungee jumper
Consider a bungee jumper, and find the energy conversions
Gravitational PE
KE + PE
Elastic PE
Consider a bungee jumper, and find the energy conversions
PE = 50 J
KE = ?
PE = 40 J
Elastic PE =
Conversion of energy in bouncy ball
Does the ball have potential
energy when it is on the floor?
Why does a dropped ball have potential
energy on the ground but one that is just
sitting there does not?
If the ball is perfectly elastic
10 J PE
10 J KE
10 J PE
10 J PE
10 J KE
Energy is often lost as heat
10 J PE
8 J PE
10 J KE
8 J PE
8 J KE
2 J HEAT
Note that even though the ball lost
energy, it is not destroyed
10 J PE
8 J PE
10 J KE
8 J PE
8 J KE
2 J HEAT
How does an impact convert
kinetic energy into “heat”?
Picture a small cube of atoms
which is dropped.
When friction or an inelastic collision occurs,
the temperature of the atoms in the objects
increases
The kinetic energy of the moving object, is
converted in to kinetic energy of the vibrations
of individual atoms
THE TOTAL ENERGY IS CONSTANT
THE TOTAL ENERGY IS CONSTANT
COASTER APPLET
THE TOTAL ENERGY IS CONSTANT
PE
PE
KE
PE
PE
KE
KE
KE
KE
If there is no friction it is just an exchange between KE & PE
Where will the car be moving the slowest?
PE
KE
PE
KE
Where will the car be moving the FASTEST?
PE
KE
KE
At what spots on the ride will the car
have the same speed as it does now?
Anytime it is at the same height?
PE
KE
Why might this rollercoaster designer be fired?
Let’s go
Uh oh
PE
PE
KE
COASTER APPLET
KE
Friction does not destroy the energy, it just
converts it to heat so will not go as high
PE
PE
PE
KE
KE
Energy
converted to
heat in red
PE
KE
KE
Because of friction the hills must be lower as the
ride goes on. (unless energy is added)
Let’s go
COASTER APPLET
Which ball is moving faster when it
hits the ground?
A ball is thrown straight up at 5 m/s, how fast is
it going when it reaches your hand again?
10 J PE
10 J KE
10 J KE
Which hits the ground with a
greater velocity.
A ball which is thrown up or
a ball which is dropped from
the same height?
Conservation of Energy
Initial Energy = Final Energy
PE1 + KE1 = PE2 + KE2
A .25 kg ball is rolling at 2.0 m/s towards a ramp.
How much PE will it have at its highest point?
How high above the ground will it reach?
height?
A 25 kg rock at rest slides down the
frictionless incline pictured below.
5.0 m
6.0 m
Howfast
much
does
with?
KE
does ititatstart
end
How
is itPE
moving
the with?
bottom?
Compare the speeds of the
two rocks when they reach
the bottom?
5.0 m
6.0 m
5.0 m
10.0 m
A bow is drawn back with a force of
320 N for 0.50 m.
How much work was done on the bow?
How much elastic PE is stored in the bow?
How fast will a 0.028 kg arrow leave the
bow when shot?
A typical candy bar has an energy content of
about 270 Calories or 1,129,000 J.
If you are 60 kg and want to work off the
energy by climbing stairs, how high will you
have to climb?
SHOW WORK WHERE APPLICABLE
Book Problems 119-121
Conservation 8, 10, 36, 42, 44, 45
I.)
3.0 kg
II.)
12 m
2.0 kg
5.0 m/s
h
Answers +1: 1.3, 7.5 15
v=?
Two magnets are attracted to each other.
If they are released and slam into each other,
Into what forms was the energy converted
PE
KE
Heat
Atoms behave like tiny attracted magnets
It takes energy to pull them apart,
this is like breaking a chemical bond.
Atoms behave like tiny attracted magnets
When they come back together
(forming a new bond)
They release this energy
The energy atoms release when they
form bonds through chemical reactions
is called
Chemical Potential Energy
You hop in the car and go for a ride and
then come to a stop.
braking
Chemical
PE
KE
Heat
Heat
Sound
electricity
Stereo, fan,
windshield
wipers,
headlights
If you pick up a physics book. (if you haven’t just pretend)
You increased the PE of the book,
where did that energy come from?
Where did you get it from?
Where did the food get it from?
Thanks, I was a
little chilly
4 H atoms
Earth
1 He atom
Light
Nuclear
Heat
Mmm that’s good corn
Heat
Kinetic Energy
Chemical Energy
Chemical Energy
Energy Conversion WS
Review sheet,
Have students work answers on the board
(groups) for bonus next day.
After test read
pages 111-118
answer questions page 120
#’s 14 - 20