Work, Machines, and Energy HS Physical Science Hull Middle School.
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Transcript of Work, Machines, and Energy HS Physical Science Hull Middle School.
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Work, Machines, and EnergyHS Physical Science
Hull Middle School
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Work and Power
Work-The transfer of an energy to an object by using a force that causes the object to move in the direction of the force.
W=F x d
Work (in joules)= Force (in newtons) times distance (in meters)
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Work involves a transfer of energy.
Force and work are not the same thing!
Applying a force does not always mean work. For example, you can push on a large boulder. If the boulder does not move, then work has not been done.
If the boulder were to move, then work has been done.
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Work, defined…
In order for work to have been done, the object has to move in the same direction as the force applied.
See p. 211 in textbook. Discuss the examples.
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Different amounts of force might be needed to do the same amount of work. See diagram on p. 212 (discuss).
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Vocabulary
Joule- Unit used to express energy; equivalent to the amount of wonk done by a force of 1 N acting through a distance of 1 m in the direction of the force (symbol, J)
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Vocabulary
Power-rate at which work is done, or energy is transferred.
Watt-unit used to express power; equivalent to joules per second (W).
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See p. 214. Solve the power problem and discuss.
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Calculating Power
P=W/t
Power is measured in watts. One watt is 1J/s.
Work is measured in Joules.
Time is measured in seconds.
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Summarize
1. In scientific terms, work is done when a force causes an object to move in the _____________ of the force.
2. Work is calculated as _________ times __________.
3. The unit of work is the newton-meter, or _______.
4. _______ is a measure of how fast work is done.
5. Power is calculated as ______ divided by ______.
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PRACTICE
Use W=fd
1. A crane uses an average force of 5,200 N to lift a girder 25 m. How much work does the crane do on the girder?
2. An apple weighing 1 N falls a distance of 1 m. How much work is done on the apple by the force of gravity?
3. A bicycle’s brakes apply 125 N of frictional force to the wheels as the bike moves 14.0 m. How much work do the brakes do?
4. A mechanic uses a hydraulic lift to raise a 1,200 kg car 0.50 m off the ground. How much work does the lift do on the car?
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Answers
1. 1.3 x 10 (fifth) J
2. 1 J
3. 1,750 J
4. 5,900 J
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Machines
Machine- A device that makes work easier by changing the size or direction of the force.
Work Input-work done on a machine.
Work Output-work done by a machine.
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Simple Machines Video
http://www.youtube.com/watch?v=jGkQ64zn1j0
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Calculating Work Input and Output
Work Input-The product of the input force and the distance through which force is exerted.
Work Output-The product of the output force and the distance through which force is exerted.
W=F x d
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What does a machine do to forces?
Machines can either change the size or direction of the force, but not both…
A MACHINE DOES NOT CHANGE THE AMOUNT OF WORK THAT YOU DO!
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FORCE AND DISTANCE TRADE-OFF
If a machine changes the size of the force, it also has to change the distance through which the force is exerted.
Force or distance will increase, but not both.
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Mechanical Advantage
Mechanical Advantage-a number that tells how many times a machine multiplies force.
MA= output force/input force.
Note that this is force, not work.
What is the mechanical advantage of a machine that puts out 25N of force, when you only put in 5N of force?
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Look at the illustration on p. 219.
How does a nutcracker work?
How does a hammer work?
How does a pulley work?
How does a screwdriver work?
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Mechanical Efficiency
Mechanical Efficiency-A percentage, that measures work output to work input.
ME= (work output/work input) x 100
An ideal machine would have 100% efficiency, but this just doesn’t happen…
Output will always be less than input, because some friction must be overcome.
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Summarize!
1. A machine makes work easier by changing the _____ or ______ of the force.
2. A machine can increase _____ or _______ , but not both.
3. ________ _________ tells how many times a machine multiplies force.
4. ______ _______ is the comparison of a machine’s work output compared to work input.
5. Why are machines never 100% efficient?