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Chapter 12 Work and Machines
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Chapter 12
Section 1 - What is work?
Work is done on an object when the object moves in the same direction in which the force is exerted.
Work - The product of force and distance when a force is used to move an object.
No work without motion
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Section 1 - What is work?
The amount of work done on an object can be determined by multiplying force times distance.
Work = Force x Distance work units - Joules (J)
Chapter 12
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Section 1 - What is work?
Power is the rate at which work is done.Power equals the amount of work done
on an object in a unit of time.
Chapter 12
Time
WorkPower DistanceForceWork
Time
ForcePower
Distance
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Section 1 - What is work?
Power = Joules/second = watt (W)kW = 1,000 W1 horsepower = 746 W
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Chapter 12
Section 2 -How Machines Do Work
Machine - A device that changes the amount of force exerted or
the direction in which force is exerted. A device with which you can do work in a way that
is easier or more effective.
A machine makes work easier by changing at least one of three factors. A machine may change the amount of force you exert, the distance over which you exert your force, or the direction in which you exert your force.
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Chapter 12
Section 2 -How Machines Do Work
pg.415
Increase force, decrease distance
Increase distance, decrease force
Change direction
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Section 2 -How Machines Do Work
A machine’s mechanical advantage is the number of times a machine increases a force exerted on it.
Chapter 12
ForceInput
ForceOutput (MA) Advantage Mechanical
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Section 2 -How Machines Do Work
Friction reduces mechanical advantageThe efficiency of a machine compares the
output work to the input work.Efficiency is expressed as a percent.Efficiency = output work/input work x
100%Problems on page 418
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Chapter 12
Section 3 - Simple Machines
Six types of machinesInclined planeWedgeScrewLeverWheel & axlePulley
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Chapter 12
Inclined Plane
An inclined plane is a flat sloped surface.Less force exerted over longer distance
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Inclined Plane
You can determine the ideal mechanical advantage of an inclined plane by dividing the length of the incline by its height.
Ideal Mechanical Advantage= Length of incline/ Height of incline.
Chapter 12
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Chapter 12
Wedge
A wedge is a device that is thick at one end and tapers to a thin edge at the other end.
Less force exerted over longer distance
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Wedge
The ideal mechanical advantage of a wedge is determined by dividing the length of the wedge by its width.
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Chapter 12
Screw
Inclined plane wrapped around a cylinderLess force
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Screw
The ideal mechanical advantage of a screw is the length around the threads divided by length of the screw.
Chapter 12
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Chapter 12
Lever A lever is a rigid bar that is free to pivot, or rotate, on a fixed point. Fulcum - the fixed point that a lever pivots around
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Lever
The ideal mechanical advantage of a lever is determined by dividing the distance from the fulcrum to the input force by the distance from the fulcrum to the output force.
Ideal mechanical advantage = Distance from fulcrum to input force / Distance from fulcrum to output force
Chapter 12
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Chapter 12
3 Types of Levers
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Wheel and Axle
A wheel and axle is a simple machine made of two circular or cylindrical objects fastened together that rotate about a common axis.
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Chapter 12
Wheel and Axle
Less force exerted over longer distanceDoorknob
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Wheel and Axle
You can find the ideal mechanical advantage of a wheel and axle by dividing the radius of the wheel by the radius of the axle.
Mechanical advantage = Radius of wheel / Radius of axle
Chapter 12
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Chapter 12
PulleyA pulley is made of a grooved wheel with rope or cable wrapped around it.Change amount and direction of input force
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Pulley
The ideal mechanical advantage of a pulley is equal to the number of sections of rope that support the object.
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Chapter 12
Section 4 - Machines in the human body
Lifting levers - most of the machines in your body are levers that consist of bones and muscles.
Pg.432 Levers in the bodyWedges - front teeth (incisors)Break toothpick in fingers
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Compound Machines
A compound machine is a machine that utilizes two or more simple machines.
The ideal mechanical advantage of a compound machine is the product of the individual ideal mechanical advantages of the simple machines that make it up.
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Chapter 12
Ch. 12 Vocabulary
work, machine, efficiency, mechanical
advantage, inclined plane,
wedge, screw, lever, fulcrum, wheel and axle, pulley