Transcript Physics TAKS Review

Physics TAKS Review
The stuff your government wants
you to know as a matter of
national security
Speed
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The rate at which an
object moves from one
point to another.
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Speed = Distance/time
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s=d/t
Questions
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If it takes you three hours to reach Ft.
Stockton which is 250 mile away, are you
breaking the speed limit? (speed
limit=70mi/h)
Yep, your speed is 83 mi/h.
If you travel to Ft. Stockton at a speed of 70
mi/h how long will it take?
3.6 hours (roughly 3 hours 36 min)
Can you handle the extra 36 min?
Acceleration
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The rate at which an object changes its speed.
Speeding up or slowing down
Acceleration = change in speed / time
a=(sf-si)/t or ∆s/t
Measured in meters per second per second (which
is written as m/s2)
Questions
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If a Ferrari can go from 10 m/s to 40 m/s in 2.0 s
what is it’s rate of acceleration.
Δs = 40m/s – 10 m/s = 30 m/s
t = 2.0 s
a = Δs/t = (30 m/s)/2.0 s = 15 m/s2 (It is speeding up
at a rate of 15 meters per second each second!)
Sounds fun, yea?
Until you see a cop. Then you’ll have a NEGATIVE
acceleration. What does that mean?
Questions
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If you punch the gas on a Toyota Corolla it
will accelerate at a lazy 2.5 m/s2. How many
seconds does it take to reach a speed of 20
m/s if it starts from rest. (What is rest??)
Δs = 20m/s – 0.0 m/s = 20 m/s
a = 2.5 m/s2
a = Δs/t → t = Δs/a = (20 m/s)/(2.5 m/s2) = ?
t = 8.0 s
Acceleration of Gravity
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When you drop something it accelerates as it falls.
And it doesn’t matter what you drop (a marble, a
Toyota, some bloke named Galileo) they all
accelerate at the same rate.
This is the acceleration of gravity and it’s equal to
9.8 m/s2 (here on Earth)
That means every second something falls it
increases its speed by 9.8 m/s.
After falling for two seconds an object would have a
speed of about 20m/s (9.8m/s2 x 2.0s)
Acceleration of Gravity
Reality Check
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WAIT A MINUTE!!
If you drop a feather and a bowling ball off the
leaning tower of Pisa at the same time, they will not
accelerate at the same rate! o_O
This is because the feather is slowed down because
of air resistance. It has trouble pushing its way
through all those air molecules on the way down.
But if you remove the AIR from the city of Pisa and
drop the feather and the bowling ball. They will both
accelerate at the same rate, 9.8 m/s2 
This is not a science project I would recommend :-\
Mass
(‘inert’ia)
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Inert means idle, or LAZY
In Physics it is better to think of mass in the
way that it influences motion so we
sometimes call it inertia. (key word ‘inert’)
Everything that has mass is LAZY! The more
mass it has, the lazier it is!
Inertia is how much an object does not want
to change how it is moving. Inertia is how
much it wants to be inert.
Mass
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Smaller masses will
change velocity easily
because they have less
inertia.
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Larger masses do not
change their velocity
easily because they
have more inertia
Newton’s Laws of Motion
1st Law
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All this talk of mass or inertia naturally leads us to Newton’s
three laws of motion.
1st Law – Objects in motion tend to stay in motion and objects
at rest tend to stay at rest, unless acted upon by an outside
force. They will maintain the same velocity until acted upon.
Pretty simple yea?
2nd law of motion
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The second law relates how much force is
required to change the motion of a certain
mass.
More force is required to accelerate a given
mass a lot.
And more force is required to give large
masses a certain acceleration.
The second law is an equation: F=ma
2nd Law Questions
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How much force is required to accelerate a
10 kg mass by 2.5 m/s2?
F=ma=(10kg)(2.5m/s2) = 25 N
Force is measured in Newton’s
How much would a 5 kg object accelerate
under the same force?
a=F/m=(25N)/(5kg)=5.0m/s2
Twice as much acceleration because ½ as
much inertia
3rd Law
(proof of karma)
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Every force has an equal and opposite force.
If you push on an object.
it pushes back on you.
They are called the action and the reaction.
F(A→B) = -F(A←B)
3rd Law cont.
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In the previous picture both skaters had the same mass so
they accelerated by the same amount and had the same
velocity in the end.
If the masses are different they still put the same force on
each other, but the larger mass will accelerate the least
because of Newton’s 2nd Law. It’s a heavier mass, so it
accelerates less.
3rd Law question
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A person jumps off a diving board and the
Earth puts a force of gravity downward on
them of about 750 N. Does this mean that
they also pull upward on the Earth with 750 N
as they fall?
Yep. This force causes the person to
accelerate at 9.8 m/s2 downward but the
same force on the Earth gives it negligible
acceleration upward. The Earth has a lot of
inertia!
Force of Gravity
(AKA Weight)
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A force you probably experience more than
any other force is the force of gravity.
The force of gravity is also called ‘weight’.
Weight is the amount of force that a mass
applies downward (like on a bathroom scale)
because of gravity
It depends on the mass of the object and the
acceleration of gravity.
What is the Unit for Force?
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In the United States, force is measured in
pounds. In the rest of the world (and in
Science) it is measured in Newtons.
Since F=ma, with mass in kilograms and
acceleration in meters per second squared…
.
The unit for force is a kg m/s2
OR
A NEWTON!
Force of Gravity
Questions
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If your mass is 70 kg, what is your weight on
the planet Earth?
(70kg)(9.8m/s2)=690N
What is your weight on the Moon, where the
acceleration of gravity is 1.7m/s2?
(70kg)(1.7m/s2)=120N
How massive would you be on Earth if you
had a weight of 120N?
(120N)/(9.8m/s2)=12kg
Work & Energy
an alternative way of viewing motion
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One of the simplest forms of energy is kinetic energy
or energy of motion.
When an object is moving it is said to posses a
certain amount of kinetic energy that depends on
how fast it is moving.
The faster an object moves the more kinetic energy
it has.
Kinetic energy = KE = ½ mv2
Kinetic Energy is generally measured in Joules.
Work
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Work is a transfer of energy into or out of an
object.
Think about when you do work. It causes
you to lose energy because the energy you
had has gone elsewhere.
In order for work to be done, a force has to
be applied to an object and the object has to
move a distance.
W=Fd (work equals force times distance)
Work and Kinetic Energy
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Work is measured in joules, just like kinetic energy
is measured in joules.
When work is done to an object it either gains or
loses its KE. (speeds up or slows down)
W=KE2 - KE1
OR
W= ΔKE
Questions
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If you push on a wall are you doing work?
Not unless the wall moves somewhere or changes
its kinetic energy (speeds up or slows down).
If you put a 40 N force on a cart to push it 3.0 m.
How much work did you do?
W = Fd = (40 N)(3.0 m) = 120 J
How much kinetic energy did you give the cart?
120 J
About those 120 Joules in the
last slide
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Sometimes an object isn’t moving (therefore no KE)
and you push on it and move it a distance (therefore
you did work) but afterward it’s still not moving (still no
KE).
You might think, “I did work! I transferred energy!
Shouldn’t it’s K increase? Shouldn’t it be moving
afterward?”
Well, friction also did work, but in the opposite way.
So all of the energy you gave the object was taken
away by friction. Friction transferred that energy back
out of the object.
Friction always does work to take energy out of things.
Darn that friction!
Power
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Power is the rate at which work is done.
If you do a certain amount of work fast, you have a
lot of power.
If you do it slow you have little power.
P=W/t (power is measured in Watts)
Questions
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How much work does a 100 W light-bulb do
in 1.0 min
P=100 W, t = 60 s
P = W/t → W = Pt = (100 W)(60 s) = 6000 J
If you use a different light bulb that puts out
the same amount of light but only has a
power of 25 W, how much energy do you
save in that minute?
4500 J because you only use 1500 J.
Gravitational Potential Energy
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Sometimes an object can have energy in it but it
isn’t moving. For example: a book high up on a
shelf.
If the book falls it gets faster and faster on the way
to the ground. It’s kinetic energy increases, but
where did that energy come from?
Work was done on the book by the force of gravity.
Gravity transferred energy from a stored form called
gravitational potential energy and turned it into
kinetic energy.
Gravitational Potential Energy
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Gravitational potential
energy is written with the
variable PE.
The more height (h) an
object has the more PE it
has.
Larger masses can hold
more potential energy.
PE=mgh (g = 9.8m/s2)
Potential energy is
measured in Joules like any
type of energy
Questions
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What has more potential energy, A 20.0 kg object
10.0 m from the ground or a 5.00 kg object 20.0 m
from the ground?
PE1=mgh=(20.0kg)(9.80m/s2)(10.0m)=1960J
PE2=(5.00kg)(9.80m/s2)(20.0m)=980J
20kg wins!!
How high would the 5.00kg mass need to be to have
as much potential energy as the 20.0kg mass?
PE=mgh→h=PE/(mg)
1960j/(5.00kg x 9.80m/s2)=40.0m
2 Useful Energies and One Not
So Useful Energy
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So far we have talked about two types of energy.
Do you remember what they are?
Gravitational Potential Energy and Kinetic Energy
There are actually several other forms of potential
energy like the energy you can store in a spring or a
battery or the energy stored in the food you eat. But
at this point you only need to know gravity’s
potential energy.
Kinetic energy only comes in one form.
There is one other form of energy. Do you know
what it is?
Thermal Energy
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Thermal energy is just a bunch of kinetic and potential
energies at the level of molecules and atoms.
However, those molecules and atoms move around
with this energy in very random ways, being mostly
useless.
OK, not completely useless. You can use it to keep
you warm and to drive chemical reactions. So I guess
it’s useful in those ways.
It can also be turned into potential or kinetic energy by
using a heat engine like the one in your car.
But it’s tricky, and you can never get at all of it. Once
energy becomes thermal energy, it’s pretty much ‘lost’.
More on thermal energy later.
Energy is Conserved
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As an object falls it gets faster or gains kinetic
energy.
It gets that kinetic energy from the potential energy it
had.
This happens the other way too.
If a ball is moving upward into the air it slows down.
It’s potential energy is increasing because it’s kinetic
energy is decreasing.
Simply put, energy never just disappears. If you
lose it as one form you will gain it as another form.
Energy is Conserved
Question 1
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A ball has 20 J of potential energy while
sitting still (K=0 J) at the top of a hill. It starts
rolling down the hill and soon has only 5 J of
potential energy because of its change in
height. How much kinetic energy does it
have?
15 J
It lost 15 J of potential energy and gained 15
J of kinetic energy.
Question 2
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Imagine a book sliding down
an incline with 20j of KE and
15j of PE at point A.
(KE + PE = 35j)
Because of friction the book
slows to a stop at a lower point
(B) where there is only 5 j of
potential energy.
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How much kinetic and potential energy does it have now?
KE=0j
PE=5j
KE + PE = 5j
Where’d the other 30j go? Energy is conserved right?
How much thermal energy was created by friction?
30 j
Simple Machines
Simple Machines
(making work easier, not less)
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Suppose you have to lift a 50 kg
object upward 2.00m. How much
work will you have to do?
You’re lifting a mass against the
force of gravity (AKA weight,
Fg=mg) so you have to supply as
much force as the force of gravity to
lift it. (mg=490 N)
You’re lifting it 2.0 m so work is
being done
W = Fd
(490N) (2.0m) = 980j
490N is not small potatoes. That’s
a lot of force to have to apply.
Especially if you haven’t been
working out :-/
Simple Machines
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This is where a simple machine like a lever or a
system of pulleys would be useful.
A simple machine allows you to use less force to do
a certain amount of work (W=Fd).
The trade off is that you put the smaller amount of
force over a longer distance.
So basically, you input a small force over a long
distance and the simple machine outputs a large
force over a short distance. See the next slide for
some examples.
Simple Machine Examples
How many simple machines can
you see in this corkscrew?
Simple Machines
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Although you don’t have to exert as much force you will end up
having to do more work. It will take more of your energy to
complete the task with a simple machine.
This is because no machine can perfectly transfer your input
work to the output side of the machine. There is always some
loss of energy as thermal energy.
If you think about it, it kind of makes sense. When have you ever
gotten as much out of something as you put into it.
However, the extra energy needed isn’t that bad because the
input force is less, which makes the job easier.
Lets Talk a Little More About
Thermal Energy
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Heat (thermal energy) can move from one
place to another in 3 ways.
Conduction (by contact – matter is needed to
transfer it)
Convection (by circulating – matter is needed
to transfer it)
Radiation (like warmth from the sun – matter
is NOT needed to transfer it)
Conduction
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Heat energy causes particles to move faster
Those particles collide with and energize the
ones next to them… which energize the ones
next to them, and so on
Kind of like hitting balls on a pool table
Convection
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When the particles in a gas or a liquid are heated,
they move faster and spread out. Therefore, the gas
or liquid becomes less dense and rises.
As those particles move farther from the heat
source, they cool and slow down. The fluid
contracts, becoming more dense, and sinks.
Radiation
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When atoms jostle around with their
thermal energy as they do, they
create an electromagnetic
disturbance in the space around
them.
This disturbance is a lot like light. It
can move at the speed of light and
can move through empty space.
Eventually the disturbance will reach
other atoms and cause them to
jostle around too.
Therefore, the thermal energy has
traveled through empty space from
one spot to another.
This is how the warmth gets to us
from the Sun.
Waves
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An oscillation is any motion that repeats itself.
Essentially any object that moves back and
forth is in oscillation
If that object is attached to other objects
around it then the oscillation will travel
through the objects.
This is called a wave.
Basic Parts of a Wave
When Waves Collide…
OOPS! I Mean Interfere.
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When two waves head toward each other
and they are both peaked or both troughed
they make one big wave.
This is called constructive interference.
When two waves head toward each other
and one is peaked and the other is troughed.
they can cancel completely
This is called destructive interference.
Have a look at the next slide.
Interfering Waves
More Interfering Waves
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Here’s another representation
as waves spread out from two
sources
The sources could be two
stereo speakers or two kids
splashing in a swimming pool,
anything that makes waves.
The dark regions are where
peaks and troughs are coming
together, so destructive
interference.
I bet you can guess what’s
happening in the lighter
regions.
Transverse Wave
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In this wave, the medium (the letters) move
transverse (perpendicular) to the way the
wave moves.
The wave is moving this way ggggg
The letters move this way hihihihihi
An example of a transverse wave is light
Check this out!
www.surendranath.org/Applets/Waves/Twave
01/Twave01Applet.html
Longitudinal Wave
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In this wave, the medium (the letters) move
longitudinal (parallel) to the way the wave
moves.
The wave is moving this way ggggg
The letters move this way gfgfgfgfg
Sound is a longitudinal wave.
Look at this:
www.surendranath.org/Applets/Waves/Lwave
01/Lwave01Applet.html
Transverse Waves Can Be
Polarized
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Transverse waves can oscillate in many different
ways.
Imagine that instead of moving to the right on the
screen the wave is coming out at you.
There are many ways to be perpendicular to that.
Up and down.
Right and left.
Diagonal.
And everything in between.
Polarization
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If you can restrict all the different ways that a
transverse wave can oscillate
to just one way
that’s called polarization.
Polarizing filters can do this, like the ones on
some sunglasses. Some gems can do this
too.
Look at the next slide for some visualizations.
Polarization
OK! Now You’re Ready To Do
Some TAKS Physics.