Chapter 5 Work and Machines. Work Transfer of energy that occurs when a force makes an object move.
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Transcript of Chapter 5 Work and Machines. Work Transfer of energy that occurs when a force makes an object move.
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Chapter 5
Work and Machines
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Work
Transfer of energy that occurs when a force makes an object move.
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Work – Two Conditions
1. The object must move.2. The motion must be in the
same direction as the applied force.
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Is Work Done?
Picking up a stack of books.Holding a stack of books.Walking while holding a stack
of books.
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Work and Energy
When work is done, a transfer of energy occurs (energy = ability to do work)
Ex. When you pick up a box, you transfer energy from your muscles to the box, increasing its PE
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Calculating Work
Work = Force X distanceW=FdF = Nd = mW = N-m = J
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Power
Is the rate at which work is done.
Amount of work done in a certain amount of time.
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Calculating Power
Power = Work/timeP = W/tW = Jt = secP = J/sec = Watts (W)
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Power and Energy
Energy can be transferred without involving work.
Ex. Light bulb transfers energy into heat and light.
P = E/t
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Machines
A device that makes doing work easier.
Work by:
–Increasing F (car jack)
–Increasing d (ramp)
–Changing direction (ax)
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Work done by Machines
Two forces involved
–Effort Force (Fe) – the force applied to a machine.
–Resistance Force (Fr) – the force applied by the machine to overcome resistance.
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Work done by Machines
Two types of work involved:
–Input work (Win) – work done by you on a machine.
–Output work (Wout) – work done by the machine.
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Conserving Energy
What does the Law of Conservation of Energy state?
You transfer energy to the machine, and the machine transfers energy to an object.
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Conserving Energy
A machine cannot create energy, so Wout is never greater that Win.
Can Wout = Win ? Why?
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Ideal Machines
What is an ideal Machine?If a machine allows you to
apply less force, then how do you get the same amount of work out of the machine?
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Mechanical Advantage (MA)
It is the number of times a machine multiplies the effort force.
IMA vs AMA
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Calculating MA
MA = resistance force/effort force
MA = Fr/Fe
MA has no units. Why?
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Efficiency
A measure of how much of the work put into a machine is converted into useful output work.
Why is work output always less than work input?
How can you efficiency?
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Calculating Efficiency
Efficiency = (Wout/Win) X 100%
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Simple Machines
A machine that does work with only one movement.
Six Types: 1)lever, 2)pulley, 3)wheel and axle, 4) inclined plane, 5) screw, and 6) wedge.
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Lever
A bar that is free to pivot about a fixed point or fulcrum.
Effort arm = distance from fulcrum where effort force is applied.
Resistance arm = distance from fulcrum where resistance force is applied
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Types of Levers
Based on positions of effort force, resistance force, and fulcrum.
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Types of Levers
1st class – fulcrum is in the middle (crowbar)
2nd class – resistance is in the middle (wheelbarrow)
3rd class – effort is in the middle (baseball bat)
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IMA of Levers
IMA = length of effort arm/length of resistance arm
IMA = Le/Lr
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Pulleys
A grooved wheel with a rope, chain or cable running along the groove.
How is a pulley a modified 1st class lever?
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Fixed Pulleys
Attached to something that does not move
Only change the direction of the force
IMA = 1
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Moveable Pulleys
One end of the rope is fixed and the wheel is free to move.
Multiplies effort forceIMA = 2
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Block and Tackle
A system of pulleys made of fixed and moveable pulleys
IMA = # of supporting rope segments. Only count effort segment if effort force is in the direction of the object’s movement.
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Wheel and Axle
Machine consisting of two wheels of different sizes that rotate together.
Modified form of a lever.IMA = rw/ra
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Gears
Modified Wheel and Axle with two wheels of different sizes with interlocking teeth.
Large wheel = effort gear, small wheel = resistance gear.
Larger effort gear = more turns of resistance gear = effort force
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Inclined plane
Sloping surface that reduces the amount of force required to do work.
Increases distanceIMA = length/height
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Screw
Inclined plane wrapped, in a spiral, around a cylinder.
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Wedge
Moving inclined plane with one or two sloping sides.
Changes the direction of effort force
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Compound Machines
When two or more simple machines are used together.
Examples?