Chapter 9 Section 1 Notes

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Chapter 9 Section 1 Notes

• Work, Power, and Machines

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What is Work?

• Work is done only when a force causes a change in
  motion of an object.
• Work is done by a force on an object.
• Work is calculated by multiplying the force by
  the distance over which the force is applied.
• Work Force x distance W F x d

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Work

• If the distance something moves is zero, there
  has been no work done
• Example If you are trying to push a car stuck in
  the mud it doesnt go anywhere, you have done
  no work because the distance moved is zero. But,
  you have applied a force.

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Work

• Work is measured in Joules.
• All of these units are equivalent 1 N m 1 J
  1 kg m2/s2
• You do 1 Joule of work when you lift an apple,
  which weighs 1 N, from your arms length down at
  your side to the top of your head (1 meter).

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Work sample problem

• Nick lifts a 0.150 kg sandwich 0.30 m from the
  table to his mouth. How much work does he do?
• Solution

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Power

• Power measures the rate at which work is done,
  or, how much work is done in a certain amount of
  time.
• Power work/time P W/t
• SI unit for power Watt (W)
• A watt is the amount of power required to do 1 J
  of work in 1 s.

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Power sample problem

• While rowing across the lake during a race, John
  does 3960 J of work on the oars in 60.0 s. What
  is his power output in watts?
• Solution

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Machines and Mechanical Advantage

• Machines multiply and redirect forces.
• Machines help to do work by redistributing the
  work we put into them.

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Machines

• Machines can
• Change the direction of the input force
• Increase output force by changing the distance
  over which the force is applied called
  multiplying the force.

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Machines

• You can do the same amount of work while applying
  a different force. Why?
• As force decreases, the distance increases, so
  you are doing the same amount of work.
• Machines make work easier by increasing the
  distance over which force is applied.

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Mechanical Advantage

• Mechanical Advantage (abbreviated MA) tells us
  how much a machine multiplies force or increases
  distance.
• Equation MA output force input distance
• input force output distance
• If MA gt1 multiplies the input force helps you
  move or lift a heavy object, such as a car.
• If MA lt1 increases distance and speed.

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Mechanical Advantage

• Example Find the MA of a ramp that is 6.0 m long
  and 1.5 m tall.
• Example Alex pulls on the handle of a claw
  hammer with a force of 15 N. If the hammer has a
  MA of 5.2, how much force is exerted on the nail
  in the claw?

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G Force

• G Force is a measurement of an object's
  acceleration expressed in g's. It may also
  informally refer to the reaction force resulting
  from an acceleration, with the causing
  acceleration expressed in g's.
• G force acts on all body parts, including organs,
  which are only loosely connected together

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• In 1954, John Paul Stapp experienced 46.2 g all
  for science. He was strapped into a
  rocket-powered sled on train tracks and
  decelerated from 630 mph to 0 in 1.25 seconds.
  This is the same as hitting a brick wall at
  120mph! He survived, but his eyes filled with
  blood and he was temporarily blinded in what is
  called a red out.

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Video of John Paul Stapp