Work, Energy, and Simple Machines

1. Work, Energy, and Simple Machines Review Objectives

2. Work and Energy

3. Work = change in energy

4. What is Work? • EQ: Who does more work? • Weight lifter #1 • Benches 250lbs one time over 22 inches • Weight lifter #2 • Benches 315lbs one time over 18 inches Weight lifter #1 = Weight lifter #2 130Joules

5. What is work? In science, the word work has a different meaning than you may be familiar with. The scientific definition of work is: using a force to move an object a distance (when both the force and the motion of the object are in the same direction.)

6. Work or Not? • According to the scientific definition, what is work and what is not? • a teacher lecturing to her class • a mouse pushing a piece of cheese with its nose across the floor

7. Work or Not? • According to the scientific definition, what is work and what is not? • a teacher lecturing to her class Not Work • a mouse pushing a piece of cheese with its nose across the floor

9. What’s work? • A scientist delivers a speech to an audience of his peers. • A body builder lifts 350 pounds above his head. • A mother carries her baby from room to room. • A father pushes a baby in a carriage. • A woman carries a 20 kg grocery bag to her car?

10. What’s work? • A scientist delivers a speech to an audience of his peers. No • A body builder lifts 350 pounds above his head. Yes • A mother carries her baby from room to room. No • A father pushes a baby in a carriage. Yes • A woman carries a 20 kg grocery bag to her car? No The force has to be in the same direction as the distance!

11. Formula for work Work = Force x Distance • The unit of force is newtons = N • The unit of distance is meters = m • The unit of work is newton-meters • One newton-meter is equal to one joule • So, the unit of work is a joule = J

12. Pause for a Cause Work = Force x Distance Calculate: If a man pushes a concrete block 10 meters with a force of 20 N, how much work has he done? (W = 20N x 10m) = 200 joules

13. Formula for work • F = ma W = m x a x d J s Power: Power is the rate at which work is done. The unit of power is the watt. Power = Work Time P = W T P = mad T W = m x a x d  P = W T

14. Pause for a Cause 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. Which student does the most work? Which student delivers the most power? Explain your answers.

15. 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. Yet, Ben is the most powerful since he does the same work in less time. Power and time are inversely proportional.

16. Pause for a Cause 2. 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? This problem requires you to use the formulas for force, work, and power all in the correct order. Force=Mass x Acceleration Work=Force x Distance Power = Work/Time

17. Pause for a Cause Force=Mass x Acceleration Force=10 x 2 Force=20 N Work=Force x Distance Work = 20 x 10 Work = 200 Joules Power = Work/Time Power = 200/5 Power = 40 watts

18. Practice Questions 1. Explain who is doing more work and why: a bricklayer carrying bricks and placing them on the wall of a building being constructed, or a project supervisor observing and recording the progress of the workers from an observation booth. 2. How much work is done in pushing an object 7.0 m across a floor with a force of 50 N and then pushing it back to its original position? How much power is used if this work is done in 20 sec? 3. Using a single fixed pulley, how heavy a load could you lift?

19. Practice Questions 1. Explain who is doing more work and why: a bricklayer carrying bricks and placing them on the wall of a building being constructed, or a project supervisor observing and recording the progress of the workers from an observation booth. Work is defined as a force applied to an object, moving that object a distance in the direction of the applied force. The bricklayer is doing more work. 2. How much work is done in pushing an object 7.0 m across a floor with a force of 50 N and then pushing it back to its original position? How much power is used if this work is done in 20 sec? Work = 7 m X 50 N X 2 = 700 N-m or J; Power = 700 N-m/20 sec = 35 W 3. Using a single fixed pulley, how heavy a load could you lift?Since a fixed pulley has a mechanical advantage of one, it will only change the direction of the force applied to it. You would be able to lift a load equal to your own weight, minus the negative effects of friction.

20. Practice Questions 4.Give an example of a machine in which friction is both an advantage and a disadvantage. 5. Why is it not possible to have a machine with 100% efficiency? 6. What is effort force? What is work input? Explain the relationship between effort force, effort distance, and work input.

21. Practice Questions 4. Give an example of a machine in which friction is both an advantage and a disadvantage. One answer might be the use of a car jack. Advantage of friction: It allows a car to be raised to a desired height without slipping. Disadvantage of friction: It reduces efficiency. 5. Why is it not possible to have a machine with 100% efficiency? Friction lowers the efficiency of a machine. Work output is always less than work input, so an actual machine cannot be 100% efficient. 6. What is effort force? What is work input? Explain the relationship between effort force, effort distance, and work input. The effort force is the force applied to a machine. Work input is the work done on a machine. The work input of a machine is equal to the effort force times the distance over which the effort force is exerted.

22. Practice Problems • Amy uses 20N of force to push a lawn mower 10 meters. How much work does she do? • How much work does an elephant do while moving a circus wagon 20 meters with a pulling force of 200N? • A 900N mountain climber scales a 100m cliff. How much work is done by the mountain climber? Work = Force X Distance Work = 20N X 10m Work = 200 J Work = Force X Distance Work = 200N X 20m Work = 4000 J Work = Force X Distance Work = 900N X 100m Work = 90,000 J

23. Practice Problems • How much work is done when a force of 33N pulls a wagon 13 meters? • How much work is required to pull a sled 5 meters if you use 60N of force? • Shawn uses 45N of force to stop the cart 1 meter from running his foot over. How much work does he do? Work = Force X Distance Work = 33N X 13m Work = 429 J Work = Force X Distance Work = 60N X 5m Work = 300 J Work = Force X Distance Work = 45N X 1m Work = 45 J

24. Force = Work / Distance Force = 15 J / 1 m Force = 15 N • Tommy does 15 Joules of work to push the pencil over 1 meter. How much force did he use? • Angela uses a force of 25 Newtons to lift her grocery bag while doing 50 Joules of work. How far did she lift the grocery bags? • The baseball player does 1234 Joules of work when hitting a baseball into left field. Assuming the baseball landed 100 meters away from home plate, how much force did the player use to hit the ball? Distance = Work / Force Distance = 50 J / 25 N Distance = 2 m Force = Work / Distance Force = 1234 J / 100 m Force = 12.34 N

27. Work and Energy

28. Formula for work Work = ΔE • F = ma W = maΔx Vf2 = Vi2 + 2aΔx  aΔx = Vf2 - Vi2 2 W = maΔx  W = m = Vf2 - Vi2 2 aΔx = Vf2 - Vi2 2 Or… W = mVf2 - mVi2 2 2 Kinetic Energy

29. Energy: Forms and Changes Can you give two examples

30. Nature of Energy • Energy is all around you! • You can hear energy as sound. • You can see energy as light. • And you can feel it as wind.

31. Nature of Energy • You use energy when you: • hit a softball. • lift your book bag. • compress a spring.

32. Nature of Energy Living organisms need energy for growth and movement.

33. Nature of Energy • Energy is involved when: • a bird flies. • a bomb explodes. • rain falls from the sky. • electricity flows in a wire.

34. Nature of Energy • What is energy that it can be involved in so many different activities? • Energy can be defined as the ability to do work. • If an object or organism does work (exerts a force over a distance to move an object) the object or organism uses energy.

35. Nature of Energy • Because of the direct connection between energy and work, energy is measured in the same unit as work: joules (J). • In addition to using energy to do work, objects gain energy because work is being done on them.

36. Forms of Energy • The five main forms of energy are: • Heat • Chemical • Electromagnetic • Nuclear • Mechanical

37. Heat Energy • The internal motion of the atoms is called heat energy, because moving particles produce heat. • Heat energy can be produced by friction. • Heat energy causes changes in temperature and phase of any form of matter.

38. Chemical Energy • Chemical Energy is required to bond atoms together. • And when bonds are broken, energy is released.

39. Chemical Energy • Fuel and food are forms of stored chemical energy.

40. Electromagnetic Energy • Power lines carry electromagnetic energy into your home in the form of electricity.

41. Electromagnetic Energy • Light is a form of electromagnetic energy. • Each color of light (Roy G Bv) represents a different amount of electromagnetic energy. • Electromagnetic Energy is also carried by X-rays, radio waves, and laser light.

42. Nuclear Energy • The nucleus of an atom is the source of nuclear energy.

43. Nuclear Energy • When the nucleus splits (fission), nuclear energy is released in the form of heat energy and light energy. • Nuclear energy is also released when nuclei collide at high speeds and join (fuse).

44. Nuclear Energy The sun’s energy is produced from a nuclear fusion reaction in which hydrogen nuclei fuse to form helium nuclei.

45. Nuclear Energy • Nuclear energy is the most concentrated form of energy. Most of us live within 10 miles of the Surry Nuclear Power Plant which converts nuclear energy into electromagnetic energy.

46. Mechanical Energy • When work is done to an object, it acquires energy. The energy it acquires is known as mechanical energy.

47. Mechanical Energy • When you kick a football, you give mechancal energy to the football to make it move.

48. Mechanical Energy When you throw a balling ball, you give it energy. When that bowling ball hits the pins, some of the energy is transferred to the pins (transfer of momentum).

49. Energy Conversion • Energy can be changed from one form to another. Changes in the form of energy are called energy conversions.

50. Energy conversions • All forms of energy can be converted into other forms. • The sun’s energy through solar cells can be converted directly into electricity. • Green plants convert the sun’s energy (electromagnetic) into starches and sugars (chemical energy).