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### Work2008

1. 1. <ul><li>S: Review Worksheet 5.1 </li></ul>
2. 2. Work and Power May 5, 2008
3. 3. Objectives <ul><li>Recognize the difference between the scientific and ordinary definitions of work. </li></ul><ul><li>Identify where work is being performed in a variety of situations. </li></ul><ul><li>Define and calculate work and power </li></ul>
4. 4. Work <ul><li>Work is the transfer of energy through motion. </li></ul><ul><li>In order for work to take place, a force must cause an object to move in the direction of the force. </li></ul>
5. 5. What’s work? <ul><li>A scientist delivers a speech to an audience of his peers. </li></ul><ul><li>A body builder lifts 350 pounds above his head. </li></ul><ul><li>A mother carries her baby from room to room. </li></ul><ul><li>A father pushes a baby in a carriage. </li></ul><ul><li>A woman carries a 20 kg grocery bag to her car? </li></ul>No No No Yes Yes
6. 6. <ul><li>There are two factors to keep in mind when deciding when work is being done: </li></ul><ul><li>something has to move </li></ul><ul><li>the motion must be in the direction of the applied force. </li></ul>
7. 7. Work is done on the books when they are being lifted, but no work is done on them when they are being held or carried horizontally.
8. 9. Direction of Force Direction of Motion
9. 10. Direction of motion Direction of Force
10. 11. Formula for work <ul><li>Work = Force x Distance </li></ul><ul><li>The unit of force is newtons </li></ul><ul><li>The unit of distance is meters </li></ul><ul><li>The unit of work is newton-meters </li></ul><ul><li>One newton-meter is equal to one joule </li></ul><ul><li>So, the unit of work is a joule </li></ul>
11. 12. Power <ul><li>Power is the rate at which work is done. </li></ul><ul><li>Power = Work * /Time </li></ul><ul><li>* (force x distance) </li></ul><ul><li>The unit of power is the watt. </li></ul>
12. 13. <ul><li>1 horsepower = 550 ft lbs / sec or 746 Watts </li></ul>
13. 14. Check for Understanding <ul><li>Two physics students, Ben and Bonnie, are in the weightlifting room. Bonnie lifts the 50 kg barbell over her head (approximately .60 m) 10 times in one minute; Ben lifts the 50 kg barbell the same distance over his head 10 times in 10 seconds. </li></ul><ul><li>Which student does the most work? </li></ul><ul><li>Which student delivers the most power? </li></ul>
14. 15. <ul><li>Ben and Bonnie do the same amount of work; they apply the same force to lift the same barbell the same distance above their heads. </li></ul><ul><li>Yet, Ben is the most powerful since he does the same work in less time. </li></ul><ul><li>Power and time are inversely proportional. </li></ul>
15. 16. <ul><li>How much power will it take to move a 10 kg mass at an acceleration of 2 m/s/s a distance of 10 meters in 5 seconds? </li></ul><ul><li>Force = Mass x Acceleration </li></ul><ul><li>Work = Force x Distance </li></ul><ul><li>Power = Work/Time </li></ul>
16. 17. <ul><li>How much power will it take to move a 10 kg mass at an acceleration of 2 m/s/s a distance of 10 meters in 5 seconds? </li></ul><ul><li>Force = Mass x Acceleration </li></ul><ul><li>Force = 10 kg x 2 m/s/s </li></ul><ul><li>Force = 20 N </li></ul><ul><li>Work = Force x Distance </li></ul><ul><li>Work = 20 N x 10 m </li></ul><ul><li>Work = 200 Joules </li></ul><ul><li>Power = Work / Time </li></ul><ul><li>Power = 200 J / 5 s </li></ul><ul><li>Power = 40 watts </li></ul>