section 8.1 work, power, and machines. objectives define work and power calculate the work done on...
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Section 8.1
Work, Power, and Machines
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Objectives
• Define work and power• Calculate the work done on an object and the
rate at which work is done• Use the concept of mechanical advantage and
calculate it
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Is this considered work?
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Work• Definition: A quantity that measures the effects of a
force acting over a distance• Equation: W = Force * distance• W = F * d• Measured in units of Joules (J)• No force = no work– Example: A ball rolling on a flat surface at constant
velocity• No distance = no work– Example: A weightlifter holding a huge weight overhead,
but not moving it.• Force and distance must also act parallel to each other,
so is the ant actually doing any work?
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Example
• A crane uses an average force of 5200 N to lift a girder 25 meters. How much work does the crane do on the girder?
• W = F * d• W = (5200 N) * (25 m)• W = 130,000 J
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The definition of power?
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Power
• Definition: A quantity that measures the rate at which work is done
• Equation: Power = work / time• P = W / t• Measured in units of Watts (W)• In general: Doing the same amount of work
but in less time requires more power. (running vs. walking upstairs to the 10th floor)
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Example:
• A crane uses an average force of 5200 N to lift a girder 25 meters in a time of 150 seconds. Calculate the power output of the crane.
• W = F * d• W = (5200 N) * (25 m)• W = 130,000 J• P = W / t• P = (130,000 J) / (150 s)• P = 870 W
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Mechanical Advantage
• Definition: A quantity that measures how much a machine multiplies force or distance
• Equation: • Mechanical advantage = output force / input force• Mechanical advantage = input distance / output distance
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A mathematical example:
• W = F * d• 2 J = 2 N * 1 m• 2 J = 1 N * 2 m• 2 J = 0.5 N * 4 m• 2 J = 0.25 N * 8 m• 2 J = 0.125 N * 16 m• 2 J = 0.0625 N * 32 m
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Moral of the story:• If you’re not superman, you can use mechanical advantage to lift a car by
applying small forces repeatedly over larger distances to do the same amount of work
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Example:
• Determine the mechanical advantage of an automobile jack that lifts a 9900 N car with an input force of 150 N.
• Mechanical advantage = output force / input force
• Mechanical advantage = 9900 N / 150 N• Mechanical advantage = 66
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Bonus: Rube-Goldberg
• Rube Goldberg device• Watch for these things:– Count how many times gravity pulls an object
downward.– Count how many times a given object is moving.– See if there are other ways in which energy is
being stored and/or released.