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Ch 14
Work, Power, and Machines
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• Work – transfer of energy
• through motion• a. Force must be
exerted through a distance
Ch 14 WORK AND POWER
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• b. Amount of Work depends on:
–1. Amount of applied Force
–2. Distance over which the force is applied
Work
F d
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• c. W=Fd
(Force multiplied by distance)
• d. Unit is the Joule (1J = 1Nm)
Work
F d
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• e. In order for work to be done:
–1. Something has to move
–2. Motion must be in the direction of the force
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• How much work is done when a car is pushed with 600 N of force for a distance of 50m?
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• How far will a ball roll if 17J of work was done to push it with 2 N of force?
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Power – rate of doing work
a. To increase Power:
–1. increase work done
– 2. do work in less time
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b. P=W/t • (Work divided by time)
c. SI unit is Watt
(1W=1J/s)
d. common unit is horsepower (1hp=746W)
Work
P t
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• How much power is needed to do 40 Joules of work in 5 seconds?
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• How long will it take to do generate 70 Watts of power if I do 4900 Joules of work?
• How much power will I need if I want to do the same amount of work in half the time?– Twice as much
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Reference table check
Work
F d
Work
P t
Fd
P t
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• You exert a vertical force of 72 N to lift a box a height of 1 meter in a time of 2 seconds. How much power is used to lift the box?
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Star questions
• True or False
–Decreasing the amount of time it takes to do work, increases the power
–True
–Motion must occur in order for work to be done
–True
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Machines
• Make work easier
• Device that changes a force
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How do machines make work easier?• Increasing force
–Rising a car using a jack• Increasing distance
–Using oars to row a boat• Changing direction
–Pulling back on the
oars to row a boat
More details later-this is an overview :o)
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Work Input (on) vs Work Output (by)
• Work done on a machine as the input force acts through the input distance
• Work done by a machine as the output force acts through the output distance
You cannot get more work out of a machine than you put into it!
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Mechanical advantage
• # of times the machine increases an input force
• Actual (AMA) = output force( FR)
input force (FE)
• FR = resistance (output) force
• FE = effort (input) force
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• You test a machine and find it exerts a force of 5N for each 1N of force you exert operating the machine. What is the Actual Mechanical Advantage of the machine?
• 5N/1N = 5
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• If the output force is 100 N and the input force is 15 N, what is the AMA of the simple machine?
• Output/input=100N/15N=
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• Ideal (IMA) = MA in absence of friction
• IMA = input distance( dE)
Output distance (dR)
• dE = effort (input) distance
• dR = resistance (output) distance
• bc friction is always present,
AMA is always less (<) than IMA
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• If x = 3 cm and y = 15 cm, what is the ideal mechanical advantage of the pliers?
x y
• 15cm / 3cm = 5
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• A student working in a grocery store after school pushes several carts together along a ramp. The ramp is 3 m long and rises 0.5 m. What is the mechanical advantage?
• Input/output=3m/0.5m=6
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• What is the MA?
• 12 / 3=4
12 m
3 m
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• What is the MA of the pulley?1
2
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no machine can be 100% efficient, why not?
Efficiency
work outputEfficiency = ------------------------- X 100%
work input
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Thanks Cambridge Physics Outlet for amazing graphics!
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• You have just designed a machine that uses 1000J of work from a motor for 800J of useful work the machine supplies. What is the efficiency of your machine?
800J / 1000J X 100 = 80%
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STAR Questions
• What three things make machines work easier
• Increasing force• Increasing distance• Changing direction
• What two things does the amount of work depend on? – Force & distance
• To increase Power I must do work in ________ time – less
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Types of simple machines
1. Lever – rigid bar that is free to move around a fixed point
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• Fulcrum=the fixed point the bar rotates around
• Input arm (effort) = distance between the input force and fulcrum
• Output arm (load) = distance between the output force and the fulcrum
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• Position of fulcrum identifies class of lever
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First class levers
• Fulcrum located between the input force (effort) and output force (load)
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Second class levers• Output force
(effort) is located between the input force (load) and fulcrum
• MA is >1
• Increases force
The input distance your hands move to lift the wheelbarrow is larger than the output distance the wheelbarrow moves to lift its load
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Third class levers• Input force is
located between the fulcrum and the output force
• MA <1
• Increases distance
The output distance over which the lever exerts its force is always larger than the input distance you move the lever through
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2. Wheel and Axle
• Variation of a lever
• Consists of two disks (cylinders) each one with a different radius
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3. Pulley
• Variation of a lever
• Consists of a rope that is different in size, direction, or both from that of the input force
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3 types of pulleys
1. Fixed pulley = changes only the direction of the input force
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2. Movable pulley = changes both direction and the size of the input force
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3. Pulley System= made up of both fixed & movable pulleys
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4. Inclined Plane
• Slanted surface along which a force moves an object to a different elevation
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5. wedge• Variation of inclined plane
• A V-shaped object whose sides are two inclined planes sloped toward each other
* A thin wedge has a greater IMA than a thick wedge of the same length
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6. screw• Variation of inclined plane
• Wrapped around a cylinder
*Screws w/ threads that are closer together have a greater IMA
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Identify which class lever goes with each picture
STAR Questions
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Complex (Compound) Machines
• Combination of 2or more simple machines to operate together
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• device, or apparatus is a deliberately over-engineered machine that performs a very simple task in a very complex fashion, usually including a chain reaction.
• The expression is named after American cartoonist & inventor Rube Goldberg.
FYI: Rube Goldberg machine
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STAR Questions
Wheel and axle
Pulley
Lever
Inclined Plane
Wedge
Screw