Sct. 1 Work and Power

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, while the force is being applied, 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?

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. • More power does not change the amount of work accomplished, but it changes the amount of time the work takes.

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 • HW: Jot down two items of work that you have accomplished already today. Explain how having more power would have affected the work.

8. Work and Energy • When you do work on an object, because it has moved, you increase its 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. Sherlock 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? • How much power must your truck have to haul a load of 55, 50 N logs over the mountain? The road over the mountain is 1200 meters and you must do it in 3 minutes.

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= 400 Watts • Sherlock Power = 15 J/60 sec. = .25 Watts • Brode Power = 21 J/80 sec. = .26 Watts • Brode more powerful! • Kinetic Energy • Potential Energy

11. W=f x d • W=(55x50N) x 1200 • W = 3300000 J • P=w/t • P=3300000J/180 seconds • P=18333.3 W

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

13. 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

14. 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 = OF/IF = 2000 N / 50 N = 40

15. Ways that machines make work easier. • Exert a smaller force over a greater distance to increase output force. • Screwdriver opening a paint can, wheel barrow • Exert a larger force over a smaller distance to increase output distance. • Rake • Allow you to change the direction of the input force. • Pulley

16. 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%

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

18. 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 pulley that you put in 800 J of work and it lifts with 200 J of work?

19. Efficiency or Mechanical Advantage???? • Your car gets 27 miles per gallon. • A 60 watt light bulb uses 66 watts of energy per hour. • You exert 50 N of force on a screwdriver to open a paint can lid that requires 200 N of force to break the seal. • You pull down with a force of 200 N to hoist your 250 N 10 point buck up. • Your 4 wheeler engine is hot when you’re done riding it this weekend. • Friction between the rake and the grass makes raking leaves more difficult. • You are able to lift a 2000N truck using only 60 N of force into the jack.

20. Work in > Work out • This is true in all machines because work is lost to friction. • 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?

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

22. 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.