Machines, Work, and Energy

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• CHAPTER 4
• Machines, Work, and Energy

Physics A First Course
Forces and Motion
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Investigation 4A

• Ropes and Pulleys
• Study work in and work out of a simple block and
  tackle system

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Investigation 4B

• Energy Car and Track System
• Study the work done by rubber band on the car
  when the car is launched.

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Work

• Work is done when a force is exerted on an object
  and the object moves in the direction of the
  force.

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Investigation 4B

• Suppose you use a rubber band to launch a car on
  the track. How do we account for the energy of
  motion when we launch the car from the rubber
  band?

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Work and Energy in the Car and Track System

• Work done by rubber band on car must equal the
  kinetic energy the car has when it moves.

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Work and Energy in the Car and Track System

• So
• How do we measure the work done by the rubber
  band?

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Work and Energy in the Car and Track System

• Well
• Work Force X Distance
• Force can be measured with a spring scale
• Distance can be measured with a ruler

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• BUT
• We must break the rubber bands deflection down
  into increments of deflection to approximate the
  average force exerted by the rubber band this
  is the tricky part.

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Estimating work done by rubber band

• Measure force exerted by rubber band at 1, 2, 3,
  4, and 5 cm of deflection.

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Estimating work done by rubber band

• Total work done by rubber band on car will be the
  sum of each one-centimeter increments AVERAGE
  force X distance.
• SEE CHART ON PAGE 20!!!!!!!!!!!

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Now you know a good approximation of how much
work the rubber band can do on the car when
launched with 1, 2, 3, 4, and 5 cm of deflection.

• Assume all the work done by the rubber band
  becomes kinetic energy of the car

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Work Energy Theorem

• Work done kinetic energy of car
• Work ½ mv2
• We know work done and we can measure the mass.
  Lets rearrange the equation to solve for a
  predicted velocity and then test our model to see
  if it really works!
• Velocity square root of (2 X work)/mass

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Test your model!

• Use car with one marble in the center.
• Predict the speed you think the car will have
  when launched with 1 cm, 2, 3, 4, and 5 cm of
  deflection.
• Try the experiment and compare measured speeds
  with predicted speeds.

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• CHAPTER 5
• Forces in Equilibrium

Physics A First Course
Forces and Motion
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Investigation 5A

• Explore forces exerted on a ring when it does not
  move
• Explore the relationship between mass and weight

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Investigation 5B

• Explore friction!
• Using the sled on a level track, what is the
  force of friction that acts on the sled to oppose
  its forward motion? What happens when you
  increase the mass of the sled?

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Friction

• This is how we will find the force of friction
• Launch the sled and find the kinetic energy, and
  measure how far it moves before coming to a stop.
  Since the work done AGAINST friction is the force
  of friction multiplied by the distance the sled
  moves, the force of friction is therefore the
  kinetic energy (equals work) divided by the
  distance the sled moves before coming to a stop.
• (1/2mv2)/distance force of friction
• Collect data for four different sled masses!
• What happens to the force of friction when you
  increase the sleds mass?????