Work and Energy

1. Work and Energy Physical Science Chapter 13

2. Work • Examples? • Scientific definition: Work is the transfer of energy through motion. • In order for work to take place, a force must be exerted through a distance. Physical Science chapter 13

3. Work • In order for work to be done, there has to be motion, and the motion has to be in the direction of the applied force. Physical Science chapter 13

4. Work Equation • Work, like energy, is measured in joules. 1 J = 1 N ∙ m. Physical Science chapter 13

5. Example • A student’s backpack weighs 10 N. She lifts it from the floor to a shelf 1.5 m high. How much work is done on the backpack? Physical Science chapter 13

6. You try • A dancer lifts a 400-N ballerina overhead a distance of 1.4 m and holds her there for several seconds. How much work is done on the ballerina? Physical Science chapter 13

7. You try • A carpenter lifts a 45-kg beam 1.2 m high. How much work is done on the beam? • Remember that weight equals mass times acceleration due to gravity. Physical Science chapter 13

8. Power • Power is the rate at which work is done. Physical Science chapter 13

9. Watts • Power is measured in watts, named after James Watt, who invented the steam engine. • 1 W = 1 J/s • Very small unit, so we often use kW. Physical Science chapter 13

10. Discuss • Define work and power. How are work and power related? • Determine if work is being done in the following situations: • Lifting a spoonful of soup to your mouth • Holding a large stack of books motionless over your head • Letting a pencil fall to the ground Physical Science chapter 13

11. Power • Power is the rate at which work is done. Physical Science chapter 13

12. You try • While rowing across the lake during a race, John does 3960 J of work on the oars in 60/0 s. What is his power output in watts? Physical Science chapter 13

13. You try • Anna walks up the stairs on her way to class. She weights 565 N, and the stairs go up 3.25 m vertically. • If Anna climbs the stairs in 2.6 s, what is her power output? • What is her power output if she climbs the stair in 10.5 s? Physical Science chapter 13

14. Machine • A device that makes work easier Physical Science chapter 13

15. Work and machines • Work is done when a force is exerted through a distance • Machines make work easier by changing the size or direction of the force, or both. • Opening a paint can with a screwdriver • Changes size – you can use less force • Changes direction Physical Science chapter 13

16. Mechanical advantage • The number of times a machine multiplies the effort force Physical Science chapter 13

17. Examples • Find the mechanical advantage of a ramp that is 6.0 m long and 1.5 m tall. • Alex pulls on the handle of a claw hammer with a force of 15 N. If the hammer has a mechanical advantage of 5.2, how much force is exerted on the nail in the claw? Physical Science chapter 13

18. Conservation of energy • You can never get more work out of a machine than you put in • If force increases, distance must decrease. • Machines often allow you to use less force, but require you to exert that force over a larger distance. Physical Science chapter 13

19. Discuss • Describe how a ramp can make lifting a box easy without changing the amount of work that can be done. Physical Science chapter 13

20. Simple machine • A device that does work with only one movement • There are six types. • They can be divided into two families • The lever family • Simple lever • Pulley • Wheel and axle • The inclined plane family • Simple inclined plane • Wedge • Screw Physical Science chapter 13

21. Levers • Examples • Crowbars • Seesaws • Baseball bat Physical Science chapter 13

22. Definitions • A lever is a bar that is free to pivot, or turn, about a fixed point. • A fulcrum is the fixed point of a lever. Physical Science chapter 13

23. Effort force Output force fulcrum First class levers • The fulcrum is in the middle • Seesaw • crowbar Physical Science chapter 13

24. output force fulcrum input force Second class levers • The output is in the middle, with the fulcrum and one end and the input at the other • wheelbarrow Physical Science chapter 13

25. output force Input force fulcrum Third class levers • The input is in the middle, with the fulcrum at one end and the output at the other • Baseball bat • broom Physical Science chapter 13

26. Pulley • grooved wheel with a rope or chain running along the groove Physical Science chapter 13

27. Pulley • Acts like a first class lever • The center acts like the fulcrum • See the top of page 440 Physical Science chapter 13

28. Fr Fe Fixed pulley • Attached to something that doesn’t move • Change only the direction of a force • MA of 1 Physical Science chapter 13

29. Finput Foutput Moveable pulley • Attached to the object being moved • MA greater than 1 Physical Science chapter 13

30. Block and tackle • System of fixed and moveable pulleys • Has MA greater than one Physical Science chapter 13

31. Wheel and axle • Consists of a wheel and a shaft (or axle) that rotate together • The input force is applied to the wheel • The shaft exerts the output force • Examples: doorknob, water faucet, gears, meat grinder Physical Science chapter 13

32. Inclined plane • A ramp • Lifting something along an inclined plane means you cover more distance than lifting it straight up, but you get to use a smaller force Physical Science chapter 13

33. screw • An inclined plane wrapped in a spiral around a cylinder. • As you drive in a screw, the inclined plane slides through the wood. Physical Science chapter 13

34. Wedge • Two inclined planes placed back-to-back • Examples • Chisels • Knives • Axe blades • The material stays in place while the wedge moves through it. Physical Science chapter 13

35. Compound machine • A combination of two or more simple machines. • An axe – lever and wedge • Bike – series of wheels and axles Physical Science chapter 13

36. Discuss • Identify the kind of simple machine represented by each of the following examples. • A drill bit • A skateboard ramp • A boat oar • What class of lever is this? • It is easier to open a door by pushing near the knob than to open a door by pushing near the hinges. What class of lever is a door? Input force Physical Science chapter 13

37. Energy • Scientific definition: Energy is the ability to do work or to cause change. • Any sample of matter has energy if it can produce a change in itself or in its surroundings. • Energy comes in many forms, including • Radiant, electrical, chemical, thermal, and nuclear • Energy is measured in joules (J). Physical Science chapter 13

38. Work and Energy • Work is the transfer of energy through motion. • When 1 J of work is done on an object, 1 J of energy has been transferred to the object. Physical Science chapter 13

39. Potential energy • Stored energy • Depends on its position or condition Physical Science chapter 13

40. Gravitational Potential Energy • Potential energy of an object due to height above the earth’s surface. • The higher the object is, the more potential energy it has. Physical Science chapter 13

41. Calculating gravitational potential energy • h is relative • Often measured from the ground, but it doesn’t have to be • We can set h=0 anywhere that is convenient Physical Science chapter 13

42. Examples • Calculate the gravitational potential energy of the following. Assume h = 0 at the ground: • A 1200 kg car at the top of a hill that is 42 m high • A 65 kg climber on top of Mount Everest (8800 m high) • A 0.52 kg bird flying at an altitude of 550 m Physical Science chapter 13

43. Kinetic energy • Energy in the form of motion • Amount depends on the mass and velocity of the object. • Greater mass at the same velocity will have greater kinetic energy. • Greater velocity for the same mass will have greater kinetic energy. Physical Science chapter 13

44. Discuss • Explain the relationship between energy and work. • Explain the difference between potential energy and kinetic energy. • What is the potential energy of a 2.5 kg book held 2.0 m above the ground? Physical Science chapter 13

45. Calculating kinetic energy Physical Science chapter 13

46. Examples • Calculate the kinetic energy in joules of a 1500 kg car that is moving at a speed of 12 m/s. • A 35 kg child has 190 J of kinetic energy after he sleds down a hill. What is the child’s speed? • A bowling ball traveling 2.0 m/s has 16 J of kinetic energy. What is the mass of the bowling ball? Physical Science chapter 13

47. Chemical potential energy • The energy stored in foods, fuels, and batteries. • There must be a chemical reaction to get the energy out. Physical Science chapter 13

48. Mechanical energy • The sum of kinetic energy and potential energy Physical Science chapter 13

49. The sun • Gives energy to living things (starting with photosynthesis • Gives off energy as heat and light • Gets its energy from nuclear fusion • When small atomic nuclei combine into a larger nucleus • A type of potential energy Physical Science chapter 13

50. Discuss • What is the kinetic energy of a 0.02 kg bullet that is traveling 300 m/s? Express your answer in joules. • What is the kinetic energy of a 0.015 kg snowball that is moving through the air at 3.5 m/s? • What is the kinetic energy of an 8500 kg airplane that is flying at 220 km/h? (Make sure you convert to m/s first) Physical Science chapter 13