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Sct. 1 Work and Power

Sct. 1 Work and Power. Work Is done when a force causes an object to move. Carrying books Pushing grocery cart Lifting weights Dodge truck pulling Ford out of mud. Applying force to do work. Work is only done in direction of applied force and when object is moving.

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Sct. 1 Work and Power

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  1. Sct. 1 Work and Power • Work • Is done when a force causes an object to move. • Carrying books • Pushing grocery cart • Lifting weights • Dodge truck pulling Ford out of mud.

  2. Applying force to do work. • Work is only done in direction of applied force and when object is moving. • Lifting books off of desk = work by arms • Carrying books to class = not work by arms but by legs

  3. Calculating Work • Work = Force x distance • W = F x d • Joules = N x meters • What is the total amount of work done when a football player carries the ball with a force of 25 N for 50 meters? • HW: Jot down a note about work that you did between now and tomorrow in class. Explain how having more power would have affected the work.

  4. W = F x d • W = 25 N x 50 m • W = 1250 N x m • W = 1250 J

  5. Power • How quickly work is accomplished. • Who is more powerful? • A man who can lift 50 N of weight in 10 seconds or a woman who can lift 50 N of weight in 8 seconds? • Woman. Same weight and work accomplished, less time.

  6. Calculating Power • Power = work/time • P = w/t • Watts = Joules/second • What is your power if you do 200 Joules of work in 20 seconds?

  7. P = W/t • P = 200 Joules/20 seconds • P = 10 Joules/sec. • P = 10 Watts

  8. Work and Energy • When you do work on an object, because it has moved, you increase it’s potential or kinetic energy. • Potential Energy = energy of position/stored energy • Height above earth • Stretched rubber band • Kinetic Energy = energy of motion • All moving objects have kinetic energy • Work = objects in motion

  9. Work and Power HomeWORK. • How much work must you do to move 100, 10 N rocks a distance of 50 meters? • How far did you push the grocery cart if it required 500 Joules of work and needed 25 N of force? • How much power is needed for a Dodge truck to pull a Chevy truck from the mud in 10 seconds if it requires 4000 Joules of work? • Who has more power? Mr. Replogle lifts 15 pieces of cake (1 piece = 1 J of work) to his mouth in 60 seconds and Mr. Brode lifts 21 pieces of cake to his big mouth in 80 seconds. • What kind of energy does the cake have as it is being swallowed to the depths of Mr. Brode’s belly? • What kind of energy did the cake have when it was being held high above his mouth ready to plummet to its dark grave?

  10. w=f x d • w=1000N x 50 m • w= 50,000 J • w=f x d • 500J = 25 N x d • d= 20 m • P=w/t • P= 4000J/10 sec • P= 40 Watts • Barton Power = 15 J/1500 sec. = .o1 Watts • Brode Power = 21 J/1740 sec. = .012 Watts • Brode more powerful! • Kinetic Energy • Potential Energy

  11. Section 2 Using Machines • Machine • A device that makes doing work easier • Pencil • Lawn mower • Screw driver • Car • Computer • pulley

  12. Machines contd. • Change the way a person does work, not how much work is done. • Have: • Input work- effort put in • Output work- the work done by the machine • No machine can create work • Input work always greater than output work. • Due to friction

  13. Mechanical Advantage • The number of times a machine multiplies the input force. • MA = output force/input force • EX: A car jack can lift a 2000 N car when a person applies 50 N of force. • MA = 2000 N / 50 N = 40

  14. Work in = Work out • In an ideal machine. Not a real machine • Fin x din = Fout x dout • If a machine produces 50 N of force when 10 N are put in and the distance out is 2 m, how much distance must the machine have put in?

  15. Fin x din > Fout x dout • 10 N x din > 50 N x 2 m • din > 10 m

  16. Ways that machines make work easier. • Exert a smaller force over a greater distance to increase output force. • Car jack • Exert a larger force over a smaller distance to increase output distance. • Rake • Allow you to change the direction of the input force. • Pulley

  17. Efficiency • Ability of a machine to use all of the work in. • E = output work/input work x 100% • Efficiency is reduced by friction. • Therefore always less than 100%

  18. Efficiency Calculation • How efficient is a car that produces 100 Joules of work for every 300 Joules of fuel consumed? • E = work out/work in • E = 100 J / 300 J • E = .33 or 33%

  19. Group Work • What is the mechanical advantage (ma = force out/force in) of a wrench that you put 50 N of force into and it loosens a bolt with a force of 250 N? • What is the mechanical advantage of a rake that you must use 100 N of force to rake leaves with and the leaves require 20 N of force to move? • What is the efficiency of a car jack that lifts a car using 5000 J of work when you put 10000 J of work into the jack? • What is the efficiency of a machine that produces 500 J of work when 400 J of work is put in? Does this machine exist, and if so, what is it?

  20. Homework • Fin x din = Fout x dout • At home pick up a hammer and 2 nails. • Start nails in a board and have the same amount of nail sticking out of the board. • First nail, hold the hammer as close to the head as possible. • Second nail, hold the hammer as close to the end of the handle as possible. • Compare the force out.

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