Work & Power!!

1. Work & Power!! Chapter 14

2. Objectives • Define work and power. • Calculate the work done on an object and the rate at which work is done. • Use the concept of mechanical advantage to explain how machines make doing work easier. • Calculate the mechanical advantage of various machines.

3. Answer Now 10 Can You Define Work In Terms of Physics?? • Yes • No

4. What is Work?

5. Answer Now 0 What Is Work? • Manual Labor • Burger King • Lifting Weights

6. Work… • A force causing a change in the motion of an object • Work measures the effect of a force over a distance. • Object must be moved (distance) for work to have occurred

7. Work = Force x distance • W = F x d • When the force is opposing the direction of movement = - work Work = visual concept

8. How is Work Measured? • W=Fd • SI Units • Force = Newtons • Distance = meters • Work = Newton x meters • Work = N x m • 1 N*m = 1 Joule

9. What the Heck is a Joule? • Joules • Historical perspective: • Named in honor of physicists James Prescott Joule (1818-1889). He discovered work and non-mechanical energy could be interchangeable, which was the foundation of the law of conservation!!

10. Important Stuff!! • W = Fd • So… • 1 J = 1 N * m • 1 Joule = 1 kilogram * meter 2/ second2 • 1 N*m = 1 J = 1 kg*m2/s2

11. What is 1 Joule of Work? • 1 joule of work is done when you lift an apple from your waist to the top of your head!!

12. Example Problems!!! • Imagine a father playing with his daughter by lifting her over his head. How much work does he do each lift, assuming he lifts her 2.0 m and exerts a force of 190 N?

13. Answer Now 0 Is the following an example of work? A waiter carries a tray full of meals above his head by one arm across the room. • Yes • No

14. This is not an example of work. The force is the waiter pushing up on the tray (vertical force) and the displacement (horizontal motion) is across the room. The force is not causing the displacement so there is no work.

15. Answer Now 0 A tired squirrel (mass of 1 kg) does push-ups by applying a force to elevate its center-of-mass by 5 cm. Determine the number of push-ups which a tired squirrel must do in order to do a mere 5.0 Joules of work. Hint! ___ J = 1 Push up • 1 E1 Push ups • 10.0 Push ups • 11.0 Push ups • 1.1 E1 Push ups

16. While rowing in a race, John uses his arms to exert a force of 165 N per stroke while pulling the oar 0.8 m. How much work does he do in 30 strokes?

17. A mechanic uses a hydraulic lift to raise a 1200 kg car 0.5 m off the ground. How much work does the lift do on the car?

18. Answer Now 0 A teacher applies a force to a wall and becomes exhausted. Work was not accomplished? True or False. • True • False

19. Answer Now 0 A book falls off a table and free falls to the ground. Is this an example of work? • Yes • No

20. This is an example of work because of gravity. The force of gravity acts on the book which displaces the book towards the ground.

21. Answer Now 0 A rocket accelerates through space. Is this an example of work? • Yes • No

22. Yes, this is an example because the force of the released fuel, pushes the rocket, which causes the rocket to be displaced through space.

23. What is Power?

24. Power • Why is it more exhausting to run up stairs instead of walk? • What’s the difference between running andwalking up stairs?

25. Power • Quantity that measures the rate at which work is done. • Power = Work / time • P = W / t • Units for Power? • Watts

27. How did they come up with a Watt??? • P = W / t • Work = Joules • Time = seconds • Power = Joule / second • Therefore • Watts = 1 Joule / 1 second • 1 W = 1 J/s

28. How does your family pay for their Power??? • You pay for energy based on the amount of power that you use. • The Power company charges you so much per kilowatt that you use. Power = visual concept

29. Example Problems: • While rowing in a race, John does 3960 J of work on the oars in 60.0 seconds. What is his power output in watts?

30. Using a jack, a mechanic does 5350 J of work to lift a car 0.5 m in 50.0 s. What is the mechanic's power output?

31. Two physics students, Will N. Andable and Ben Pumpiniron, are in the weightlifting room. Will lifts the 100-pound barbell over his head 10 times in one minute; Ben lifts the 100-pound barbell over his head 10 times in 10 seconds. Explain your answers. • Which student does the most work? • Which student delivers the most power?

32. Ben and Will do the same amount of work; they apply the same force to lift the same barbell the same distance above their heads. • Yet, Ben is the “most powerful”, since he does the same work in the least amount of time. • Time and Power are inversely proportional.

33. During the Personal Power lab, Jack and Jill ran up the hill. Jack is twice asmassive as Jill; yet Jill ascended the same distance in half the time. Explain your answers. • Who did the most work? • Who delivered the most power?

34. Jack does more work than Jill. Jack must apply twice the amount of force to lift his twice as massive body up the same flight of stairs. • Yet, Jill is “more powerful” than Jack. Jill does ½ the work yet does it in ½ the time. The reduction in work is compensated for by the reduction in time.

35. Your monthly electric bill is expressed in kilowatt-hours, a unit of energy delivered by the flow of l kilowatt of electricity for one hour. Use conversion factors to show how many joules of energy you get when you buy 1 kilowatt-hour of electricity.

36. What is a Machine???

37. More Machines???

38. Machines • Which is easier, lifting a car yourself or using a jack? • Which requires more work?

39. How do Machines Do Work??? • They redistribute the work that we put into them!! • They change the direction of an input force • They can increase an output force by changing the distance over which the force is applied.

40. Output Force = = Input Force Mechanical Advantage • Quantity that measures how much a machine multiplies force or increases a distance. • Scientists or engineers use mechanical advantage to determine how to build machines. • Mechanical advantage Input Distance Output Distance

41. Mechanical Advantage!! • Machines with an mechanical advantage greater than 1 multiply forces and are used to lift heavy objects. • Machines with an mechanical advantage less than 1 do not multiply force, but increase distance and speed. • Your arms and a bat together form a machine that increases speed without multiplying force.

42. Mechanical Advantage Bigger Than 1 Multiply Forces Used to Lift Heavy Objects Mechanical Advantage Smaller Than 1 Don’t Multiply Forces Do increase distance and speed To Restate the Last Slide

43. Force and Work

44. Output Force = = Input Force • Calculate the mechanical advantage of a ramp that is 5.0 m long and 1.5 m high? • Does the ramp multiply force or increase distance and speed? • Remember!!! • Mechanical advantage Input Distance Output Distance

45. Mechanical Advantage • Tells how much a machine multiplies force or decreases distance

46. Mechanical Advantage Calculate the mechanical advantage of a ramp that is 5.0 m long and 1.5 m high. List the given and unknown values. Given:input distance = 5.0 m output distance = 1.5 m Unknown: mechanical advantage = ?

47. 2. Write the equation for mechanical advantage. • Because the information we are given involves only distance, we only need part of the full equation: 3. Insert the known values into the equation, and solve.