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. • 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 • 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 • 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 • 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 • 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 • 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 Distance Work = 15 N x 1 m Work = 15 J Power = work/time Power = 15 J/ 2 sec Power = 7.5 W
14.2 Work & Machines Chapter 14: Work, Power, and 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 forceyou 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 • 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 • 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. What kind of friction is involved here? The friction in this case would be fluid friction.
Section 14.2 Assessment • 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 • 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)? The output distance is greater than the input distance. Why might it be useful to know this? 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 • 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.
14.3 Mechanical Advantage & Efficiency Chapter 14: Work, Power, and Machines
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
Calculating Mechanical Advantage • How do you calculate IMA? Use words & an equation. • IMA = I. Distance/O. Distance • What two things do you need to calculate IMA? • Input Distance & Output Distance
Why do we calculate IMA instead of AMA? • It’s easier to measure • Does this give us the results we want? • No, it’s what we’d have if there was no friction (in a perfect world)
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 • 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 • How can 2 machines appear identical and yet not have the same AMA? They can have different amounts of friction.
Section 14.3 Assessment • What information would you use to calculate the efficiency of a machine? Work input and Work output
Section 14.3 Assessment • When is the IMA of a machine greater than 1? Whenever output force is greater than input force.
Section 14.3 Assessment • 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 • 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 • 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