work and machines chapter 3. lesson 1- work and power what is work? ➔ any time a force is exerted...
TRANSCRIPT
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Work and MachinesChapter 3
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Lesson 1- Work and Power
What is work?➔ any time a force is exerted on an object
that causes the object to move◆ the object must move in the same
direction in which the force is exerted.◆ No movement of object = no work done
no matter how much force is used
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Different directions = NO WORK...
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Some work is done.
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Most work is done.
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Work = Force X DistanceWork = (N) X (m)Work = _____ N.m
Therefore the SI unit for Work is called a JOULE. (1 J = force of 1N to move an object a distance of 1 meter)
How do you calculate work?
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90N X 30 m = 2700 J (force X Distance)
180 N X 60 m = 10800 J ( 4 times more)
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What is Work Demonstration…Worksheet 71 with
discussion
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What is Power?
★ the rate at which work is done○ Power = Work Force X Distance
Time Time
• Unit for Power is watt o 1J/s = 1 W
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Pinwheel Demo… (15 seconds)
1. Is work done on the pinwheel? Explain.
2. How are the two situations different?
3. Which situation involved more power? Explain.
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Investigating Power: pg 72
You and your partner will be completing this investigation.
Use your time wisely!
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Lesson 2Understanding Machines
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Input force
• This is the force you exert.o You exert this force over a specific distance.
(input work)
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Output Force
• This is the force the machine exertso The machine also moves a specific distance
(output work)
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Output distance and force
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What does a machine do?
❖Makes work easier by➢ changing the amount of force your exert OR➢ changing the distance over which you exert
your force OR➢ changing the direction in which you exert
your force
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Machines don’t change the amount of work you do, but they do change the way you do the work.
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Changing Force
If a machine allows you to use less input force, you must apply that force over a greater distance.
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Changing Force
Work = Force X Distance● Let’s Say: My force is 5 N and the distance is 2 m;
○ 5 N x 2 m = 10 J of work● If the amount of work stay the same, a decrease of force means an increase of distance.
● If I put 2N of force, then my distance would be 5m to equal 10 J of work.
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Changing Distance
If a machine allows you to move your input force over a shorter distance, than you need to apply a greater input force.
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Changing Distance
Largeinputforce
Largeoutputdistance
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Change Direction
• Some machines don’t change in either force or distance, they change the direction of the force.
Small input force
large input distance
Small output force
large output distance
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Mechanical Advantage
• The number of times a machine increases a force exerted on it.
• The ratio of output force to input force
Mechanical Advantage = output force input force
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What happens when you increase Force?
When the output force is greater than the input force - mechanical advantage > 1
YES
15N / 10N = 1.5
Greater
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What happens when distance increases?
• When a machine increases distance, the output force (machine’s force) is less than the input force (your force).
• Mechanical advantage is < 1
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What happens when direction changes?
• Input force = output forceo Mechanical advantage is 1.
The larger the mechanical advantage, the easier a machine makes your work.
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What is efficiency?
• The machine’s (output) work is always less than your (input) work because the machine has to overcome the force of friction.
• The less work a machine has to do to overcome friction, the more efficient it is.
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To calculate efficiency...
Output work X 100%
Input work
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Lesson 3Inclined Planes and Levers
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Simple Machines:
Inclined Planes - Flat sloped surface
Wedge - device that is thick on one end and tapers to a thin edge at the other
Screw - be thought of as an inclined plane wrapped around a cylinder.
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Levers:
• Rigid bar that is free to pivot, or rotate on a fixed point.
• That fixed point is called a fulcrum
• Types of levers are classified according to the location of the fulcrum relative to the input and output forces.
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Types of Levers: 1st Class
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Types of Levers: 2nd Class
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Types of Levers: 3rd Class
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More Examples of Levers:
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Lesson 4Putting Machines Together
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PULLEY Systems
Simple Machine made of a grooved wheel with a cable or rope wrapped around it.
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Types of Pulleys: Fixed
Changes the direction of the force but not the amount applied:
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Types of Pulleys: Movable
Decreases the amount of input force needed. It doesn’t change the direction of the force.
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Type of Pulleys: Block and Tackle
A pulley system make up of fixed and movable pulleys.
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Wheel and Axle
Two connected objects that rotate about a common axis.