Physics TAKS Review

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Physics TAKS Review

• The stuff your government wants you to know as a
  matter of national security

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Speed

• The rate at which an object moves from one point
  to another.
• Speed Distance/time
• sd/t

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Questions

• If it takes you three hours to reach Ft. Stockton
  which is 250 mile away, are you breaking the
  speed limit? (speed limit70mi/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?

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Acceleration

• 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)

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Questions

• If a Ferrari can go from 10 m/s to 40 m/s in 2.0
  s what is its rate of acceleration.
• ?s 40m/s 10 m/s 30 m/s
• t 2.0 s
• a ?s/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 youll have a NEGATIVE
  acceleration. What does that mean?

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Questions

• 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??)
• ?s 20m/s 0.0 m/s 20 m/s
• a 2.5 m/s2
• a ?s/t ? t ?s/a (20 m/s)/(2.5 m/s2) ?
• t 8.0 s

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Acceleration of Gravity

• When you drop something it accelerates as it
  falls.
• And it doesnt 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 its
  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)

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Acceleration of GravityReality Check

• 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
  -\

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Mass(inertia)

• 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.

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Mass

• Smaller masses will change velocity easily
  because they have less inertia.
• Larger masses do not change their velocity easily
  because they have more inertia

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Newtons Laws of Motion1st Law

• All this talk of mass or inertia naturally leads
  us to Newtons 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?

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2nd law of motion

• 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 Fma

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2nd Law Questions

• How much force is required to accelerate a 10 kg
  mass by 2.5 m/s2?
• Fma(10kg)(2.5m/s2) 25 N
• Force is measured in Newtons
• How much would a 5 kg object accelerate under the
  same force?
• aF/m(25N)/(5kg)5.0m/s2
• Twice as much acceleration because ½ as much
  inertia

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3rd Law(proof of karma)

• 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)

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3rd Law cont.

• 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 Newtons 2nd
  Law. Its a heavier mass, so it accelerates less.

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3rd Law question

• 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!

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Force of Gravity (AKA Weight)

• 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.

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What is the Unit for Force?

• In the United States, force is measured in
  pounds. In the rest of the world (and in Science)
  it is measured in Newtons.
• Since Fma, with mass in kilograms and
  acceleration in meters per second squared
• The unit for force is a kg.m/s2
• OR
• A NEWTON!

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Force of GravityQuestions

• 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

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Work Energyan alternative way of viewing motion

• 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.

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Work

• 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.
• WFd (work equals force times distance)

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Work and Kinetic Energy

• 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)
• WKE2 - KE1 OR W ?KE

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Questions

• 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

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About those 120 Joules in the last slide

• Sometimes an object isnt moving (therefore no
  KE) and you push on it and move it a distance
  (therefore you did work) but afterward its still
  not moving (still no KE).
• You might think, I did work! I transferred
  energy! Shouldnt its K increase? Shouldnt 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!

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Power

• 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.
• PW/t (power is measured in Watts)

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Questions

• How much work does a 100 W light-bulb do in 1.0
  min
• P100 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.

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Gravitational Potential Energy

• Sometimes an object can have energy in it but it
  isnt 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. Its 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.

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Gravitational Potential Energy

• 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.
• PEmgh (g 9.8m/s2)
• Potential energy is measured in Joules like any
  type of energy

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Questions

• 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?
• PE1mgh(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?
• PEmgh?hPE/(mg)
• 1960j/(5.00kg x 9.80m/s2)40.0m

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2 Useful Energies and One Not So Useful Energy

• 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 gravitys potential energy.
• Kinetic energy only comes in one form.
• There is one other form of energy. Do you know
  what it is?

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Thermal Energy

• 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 its 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 its tricky, and you can never get at all of
  it. Once energy becomes thermal energy, its
  pretty much lost.
• More on thermal energy later.

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Energy is Conserved

• 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.
• Its potential energy is increasing because its
  kinetic energy is decreasing.
• Simply put, energy never just disappears. If you
  lose it as one form you will gain it as another
  form.

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Energy is Conserved
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Question 1

• A ball has 20 J of potential energy while sitting
  still (K0 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.

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Question 2

• 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.

• How much kinetic and potential energy does it
  have now?
• KE0j PE5j KE PE 5j
• Whered the other 30j go? Energy is conserved
  right?
• How much thermal energy was created by friction?
• 30 j

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Simple Machines
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Simple Machines(making work easier, not less)

• Suppose you have to lift a 50 kg object upward
  2.00m. How much work will you have to do?
• Youre lifting a mass against the force of
  gravity (AKA weight, Fgmg) so you have to supply
  as much force as the force of gravity to lift it.
  (mg490 N)
• Youre lifting it 2.0 m so work is being done
• W Fd (490N) (2.0m) 980j
• 490N is not small potatoes. Thats a lot of
  force to have to apply.
• Especially if you havent been working out -/

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Simple Machines

• 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 (WFd).
• 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.

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Simple Machine Examples
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How many simple machines can you see in this
corkscrew?
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Simple Machines

• Although you dont 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 isnt that bad
  because the input force is less, which makes the
  job easier.

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Lets Talk a Little More About Thermal Energy

• 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)

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Conduction

• 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

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Convection

• 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.

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Radiation

• 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.

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Waves

• 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.

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Basic Parts of a Wave
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When Waves CollideOOPS! I Mean Interfere.

• 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.

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Interfering Waves
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More Interfering Waves

• Heres 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 whats happening in the
  lighter regions.

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Transverse Wave

• 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
  /Twave01/Twave01Applet.html

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Longitudinal Wave

• 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/L
  wave01/Lwave01Applet.html

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Transverse Waves Can Be Polarized

• 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.

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Polarization

• If you can restrict all the different ways that a
  transverse wave can oscillate
• to just one way
• thats 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.

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Polarization
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OK! Now Youre Ready To Do Some TAKS Physics.