14.1 work and power
DESCRIPTION
14.1 Work and Power. Chapter 14: Work, Power, and Machines. What is Work?. Define Work: The product of force and distance. Work Requires Motion. What condition must exist in order for a force to do work on an object? Some of the force must act in the same direction as the object moves. - PowerPoint PPT PresentationTRANSCRIPT
CHAPTER 14 : WORK, POWER, AND MACHINES
14.1 Work and Power
What is Work?
Define Work:
The product of force and distance.
Work Requires Motion
What condition must exist in order for a force to do work on an object?
Some of the force must act in the same direction as the object moves.
If there is no movement, no work is done.
Work Depends on Direction
The amount of work done on an object depends on what two things? Direction of the force Direction of the movement
Does all the force have to act in the direction of movement to do work? No
Think of an original idea where this is true.
Calculating Work
How do you calculate work?
Multiply the force acting in the direction of motion by the distance the object moves
Work = Force x Distance
Tomorrow when I am gone, you will do math practice with this formula
Units of Work
What are the two ways you can label work? Newton * Meter
force (N) x distance (m)
Joule (J)
1 N*m = 1 J
Using the Work Formula
How much work does a 25 N force do to lift a potted plant from the floor to a shelf 1.5 m high? Work = Force x Distance
Work = 25 N x 1.5 m
Work = 37.5 N*m
Work = 37.5 J
What is Power?
Define Power:
The rate of doing work.
What are two ways to increase power?
Increase the amount of work done in a given time.
Do a given amount of work in less time.
How does doing work at a faster rate affect the power required? More power is required.
Think of an original example of two things that do the same task, where one requires more power, but less time and the other requires less power, but more time. Plowing a field with a horse & chisel Plowing a field with a tractor
Calculating Power
How do you calculate power? Divide the amount of work done by the time needed to
do the work.
Power = Work/Time
What are two ways you can label power? Joules/seconds
work (J) / time (sec)
Watt (W)
You lift a large bag of flour from the floor to a 1 m high countertop, doing 100 J of work in 2 sec. How much power do you use to lift the bag of flour? Power = Work/Time
Power = 100 J / 2 sec
Power = 50 J/sec
Power = 50 W
James Watt and Horsepower
What is another unit for power? Horsepower (hp)
Compare this unit with watts.1 horsepower = 746 watts
Section 14.1 Assessment
1. How much work is done when a vertical force acts on an object moving horizontally?
No work is done, because none of the force acts in the same direction as the object.
Section 14.1 Assessment
2. A desk exerts an upward force to support a computer resting on it. Does this force do work? Explain.
No work is done, because there is no movement.
Section 14.1 Assessment
3. Two cars have the same weight, but one of the cars has an engine that is twice as powerful as the other. Which car can make it to the top of a mountain pass first?
The car w/ the more powerful engine will reach the top first.
Which car does more work to reach the pass?
Both cars do the same amount of work to reach the top.
Section 14.1 Assessment
4. You carry two heavy bags of groceries upstairs to your kitchen. Will you do more work on the bags if you carry them one at a time? Explain.
Carrying one bag at a time uses only half the force, but requires twice the distance.
The work done is the same in both cases.
Section 14.1 Assessment
5. You lift a book to a bookshelf 1 m above the floor. How much power is used if the upward force is 15 N and you do the work in 2 seconds?
Work = Force x DistanceWork = 15 N x 1 m Work = 15 J
Power = work/timePower = 15 J/ 2 secPower = 7.5 W
CHAPTER 14 : WORK, POWER, AND MACHINES
14.2 Work & Machines
Machines Do Work
Define machine: A device that changes a force.
What is the main purpose of machines? They make work easier to do.
In which ways can machines do this? Change the size of the force needed. Change the direction of the force. Change the distance over which a force acts.
Increasing Force
How can a machine increase a force? Increase the distance over which a small force is
exerted.
A small force exerted over a large distance becomes a large force exerted over a short distance.
Think of an original example of a machine that increases force: Bolt Cutters vs. Scissors
Increasing Distance
How can a machine increase distance? Exert a greater force over a smaller distance.
A machine that decreases the distance through which you exert a force, increases the amount of force required.
Think of an original example of a machine that increases distance: Gears on a bike Broom? Golf club? Bat?
Changing Direction
How can a machine change the direction of the applied force?
Think of an original example of a machine that changes direction: Levers Pry bars
Work Input & Work Output
How does the work done ON a machine compare to the work done BY a machine? Work done ON a machine is bigger than work done BY
it More work goes in than comes out.
What does friction have to do with this? Friction causes work/force to be lost. It can be lost as:
Heat Light Sound
Work Input to a Machine
Define input force: The force you exert on a machine.
Define input distance: The distance over which you exert a force.
Define work input: Work done by you.
Write an equation that uses the last three terms: Work Input = Input Force x Input Distance
Work Output of a Machine
Define output force: The force exerted by a machine
Define output distance: The distance over which a machine exerts a force
Define work output : The work done by a machine
Look at Figure 7 (pg. 419). What is the input distance? The length of the path over which the oar handle
moves (the arc)
How does the input distance compare to the output distance? It is less
Section 14.2 Assessment
1. A machine produces a larger force than you exert to operate the machine. How does the input distance of the machine compare to its output distance?
Because the output force is greater than the input force, the input distance must be greater than the output distance
This could be an example of a car jack.
Section 14.2 Assessment
2. You do 200 J of work pulling the oars of a rowboat. What can you say about the amount of work the oars do to move the boat? Explain.
The work done by the oars is less than 200 J, because some input force is always lost to friction.
The friction in this case would be fluid friction.
Section 14.2 Assessment
3. How can you increase the work output of a machine?
You can increase the work you put into a machine.
You can try to reduce the friction* Ball Bearings* Oil/Grease* Make sure the parts that touch have
smooth surfaces * …
Section 14.2 Assessment
4. When you swing a baseball bat, how does the output distance (the end of the bat) compare with the input distance (the distance your hands move)? Why might it be useful to know this?
The output distance is greater than the input distance.
You know that if you swing harder, you will increase the force at the end of the bat, making the ball go further.
Section 14.2 Assessment
5. An ad claims that a new wrench reduces the force needed to tighten a bolt. If this ad is true, what do you know about the input distance?
If the input force is reduced, then the input distance must be increased.
CHAPTER 14 : WORK, POWER, AND MACHINES
14.3 Mechanical Advantage & Efficiency
Mechanical Advantage
Define Mechanical Advantage The number of times the machine increases an input
force
Actual Mechanical Advantage
Define AMA: The ratio of the output force to the input force
Write the equation: AMA = O. Force/ I. Force
Ideal Mechanical Advantage
Define IMA: The MA of a machine in the absence of friction
Write the equation: IMA = I. Distance/O. Distance
Why is the AMA always less than the IMA? Because there is always friction
What can be done to make the AMA closer to the IMA? Use low-friction materials Ball bearings Oil/grease Stream-lining
Efficiency
Define Efficiency: The percentage of the work input that becomes work
output
Write out the equation: Efficiency = Work O. / Work I. X 100%
Why is efficiency of any machine always less than 100%? Because there’s always friction.
What are some ways to increase efficiency? Ball bearings Grease/Oil Smoother surfaces…
Can you think up a machine that would have 100% efficiency?
Section 14.3 Assessment
1. You test a machine and find that it exerts a force of 5 N for every 1 N of force you exert operating the machine. What is the AMA of the machine?
AMA = O. Force/ I. Force
AMA = 5N / 1 N
AMA = 5
Section 14.3 Assessment
2. How can 2 machines appear identical and yet not have the same AMA?
They can have different amounts of friction.
Section 14.3 Assessment
3. What information would you use to calculate the efficiency of a machine?
Work input and Work output
Section 14.3 Assessment
4. When is the IMA of a machine greater than 1?
Whenever output force is greater than input force.
Section 14.3 Assessment
5. Suppose you are an inventor in 1900. You are constructing a bicycle of your own design. What could you do to ensure your bicycle efficiently changes the work input into forward motion?
Reduce friction as much as possible…
Section 14.3 Assessment
6. You have just designed a machine that uses 1000 J of work from a motor for every 800 J of useful work the machine supplies. What is the efficiency of your machine?
E = (W.O./W.I.) x 100%
E = (800J/1000J) x 100%
E = 80%
Section 14.3 Assessment
7. If a machine has an efficiency of 40% and you do 1000 J of work on the machine, what will be the work output of the machine?
E = (W.O./W.I.) x 100%
W.O. = (E x W.I.) / 100%
W.O. = (40% x 1000 J) / 100%
W.O. = 400 J
Section 14.3 Assessment
8. A construction worker moves a crowbar through a distance of 0.5 m to lift a load 0.05 m off the ground. What is the IMA of the crowbar?
IMA = I.D. / O.D
IMA = 0.5m / 0.05m
IMA = 10
Section 14.3 Assessment
9. The IMA of a simple machine is 2.5. If the output distance of the machine is 1 m, what is the input distance?
IMA = I.D. / O.D.
I.D. = O.D. x IMA
I.D. = 1 m x 2.5
I.D.= 2.5 m
CHAPTER 14 : WORK, POWER, AND MACHINES
14.4 Simple Machines
What are the 6 types of simple machines?
Lever
Wheel & Axle
Inclined Plane
Wedge
Screw
Pulley
Levers
Define lever:
A rigid bar that is free to move around a fixed point
Define fulcrum:
The fixed point the bar rotates around
Levers are classified into 3 categories based on the locations of what?
The locations of the input force, output force, and fulcrum
Define input arm:
The distance between the input force and fulcrum
Define output arm:
The distance between the output force and the fulcrum
How do you calculate the IMA of a lever? Divide the input arm by the output arm
First-Class Levers
Where is the fulcrum located on a first-class lever? Between the input and output force
Draw and label a sketch of a first-class lever
What is the MA of a first-class lever? Greater than, equal to, or less than 1
Name some examples of simple machines that use first-class levers:
Teeter-totter Scissors
Second-Class Levers
Where is the output force located on a second-class lever? Between the input force and fulcrum
Draw and label a sketch of a second-class lever:
What is the MA of a second-class lever? Greater than 1 Requires less force, but more distance
Name some examples of simple machines that use second-class levers:
Wheelbarrow Nut cracker
Third-Class Levers
Where is the input force located on a third-class lever? Between the fulcrum and the output force
Draw and label a sketch of a third-class lever:
What is the MA of a third-class lever? Less than 1 The output distance is greater then the input distance
Name some examples of simple machines that use third-class levers: Broom Hockey stick Baseball bat Golf club
Wheel & Axle
Define a wheel and axle: A simple machine that consists of 2 disks or cylinders,
each w/ a different radius
Draw and label a sketch of a wheel and axle:
How do you calculate the IMA of a wheel and axle?
Divide the radius (diameter) of the input force by that of the output force
What is the MA of a wheel and axle?
Greater or less than one
Depends which end is the input
Name some examples of simple machines that use a wheel and axle: Steering wheel and column Screwdriver
Inclined Planes
Define an inclined plane: Slanted surface along which a force moves an object
to a different elevation
Draw and label a sketch of an inclined plane:
How do you calculate the IMA of an inclined plane? The distance along the plane divided by the change in
height
What is the MA of an inclined plane? Greater than 1
Name some examples of simple machines that use inclined planes: Moving ramp Wheelchair ramp Switchbacks on a mountain
Wedges
Define a wedge: A V shaped object whose sides are 2 inclined planes
sloped towards each other
Draw and label a sketch of an wedge:
What has a greater IMA:
A thin wedge 8 cm long?
A thick wedge 8 cm long?
Name some examples of simple machines that use wedges:
Knife blade
Zipper
Axe blade
Screws
Define a screw: An inclined plane wrapped around a cylinder
Draw and label a sketch of a screw:
What has a greater IMA:
A screw with close-together threads?
A screw with far-apart threads?
Name some examples of simple machines that use screws: Screws
Nuts
Bolts
Corkscrew
Pulleys
Define a pulley: A simple machine that consists of a rope that fits into
a groove in a wheel
How do you find the IMA of a pulley or a pulley system? It is equal to the number of rope sections supporting
the load being lifted
Fixed Pulleys
Describe a fixed pulley: Wheel attached at a fixed location Rotates in place Direction of force is changed, but the size of the force
is not
Draw and label a sketch of a fixed pulley:
What is the MA of a fixed pulley? Always 1 IF = OF (disregarding friction)
Name some examples of fixed pulleys: Flag pole Mini-blinds
Moveable Pulleys
Describe a moveable pulley: The pulley is attached to the object being moved
Draw and label a sketch of a moveable pulley:
What is the MA of a moveable pulley? Greater than 1
Name some examples of moveable pulleys: Sails on a ship Skyscraper window washers’ platforms
Pulley Systems
Describe a pulley system: Fixed and moveable pulleys together
Draw and label a sketch of a pulley system:
What is the MA of a pulley system? Greater than 1 Even larger than a moveable pulley
Name some examples of pulley system: Train car lifters
Compound Machines
Define a compound machine: A combination of 2 or more simple machines that
work together
Name some examples of compound machines: Car engine Washing machine Watch Scissors …