2. Unit 8: Energy, Heat, and Work

3. Law of Conservation of Energy Energy cannot be created or destroyed: it changes forms or is transferred. What is Energy? Energy is the ability to do work. The SI unit for energy is the joule (J).

4. Two Categories of Energy

5. Potential vs Kinetic Energy Potential energy is converted to kinetic energy http://www.pbslearningmedia.org/resource/hew06.sci.phys.maf.roll ercoaster/energy-in-a-roller-coaster-ride/ http://www.pbslearningmedia.org/resource/hew06.sci.phys.maf.roll ercoaster/energy-in-a-roller-coaster-ride/

6. *Potential energy is stored energy.* Gravitational potential energy (GPE) is energy stored by objects due to their position above the Earth’s surface (their height). GPE=mgh m=mass (kg) h=height (m) g=acceleration due to gravity (9.8 m/s/s)

7. Which has greater gravitational potential energy? 1.A 10 kg rock falling from 10 m? 2.A 100 kg rock falling from 10 m? 3.A 10 kg rock falling from 50 m? GPE=mgh https://www.youtube.com/watch?v=uOJfcT ZME0U

8. *Kinetic Energy is energy in motion.* The kinetic energy of a moving object depends on its mass and velocity. KE = ½ mv 2 KE=Kinetic Energy (J) m=mass (kg) v=velocity (m/s)

9. 10 m/s 5 m/s 10 m/s 1. A 1000 kg truck moving at 10 m/s 2. A 120 kg motorcycle moving at 5 m/s? 3. A 120 kg motorcycle moving at 10 m/s? ♦ Which has the most kinetic energy?

10. Temperature Temperature is how hot or cold something is Hot: heat flowing into you, increasing the KE of your atoms Cold: heat flowing out of you, decreasing the KE of your atoms http://phet.colorado.edu/en/ simulation/states-of-matter

11. Ice gets warmer while hand gets cooler Cup gets cooler while hand gets warmer

12. Thermal Energy Thermal Energy is the kinetic energy of a substance’s atoms 2 Ways to increase thermal energy 1) increase the temperature 2) increase the mass (number of particles)

13. Thermal Energy Which beaker of water has more thermal energy? B - same temperature, more mass 200 mL 80ºC A 400 mL 80ºC B

14. Some things heat up or cool down faster than others. Land heats up and cools down faster than water. Specific Heat

15. The specific heat of any substance is the amount of heat required to raise the temperature of one gram of that substance by one degree Celsius.

16. Specific Heat Because different substances have different compositions, each substance has its own specific heat.

17. Heat Transfer Which sample will take longer to heat to 100°C, if you use the same heating source? 50 g Al50 g Cu Al - It has a higher specific heat. Al will also take longer to cool down. Specific Heat Values (J/(kg·K)) Aluminum 920 Copper 380

18. Temperature vs Heat Energy What happens to temperature during a change of state of matter? Why?

20. Work Work is when a force causes a change of position of an object W = Fd Distance must be in direction of force! W:work (J) F:force (N) d:distance (m) 1 J = 1 N·m

21. Work

22. u Brett’s backpack weighs 30 N. How much work is done on the backpack when he lifts it 1.5 m from the floor to his back? GIVEN: F = 30 N d = 1.5 m W = ? WORK: W = F·d W = (30 N) (1.5 m) W = 45 J F W d

23. Work u A dancer lifts a 40 kg ballerina 1.4 m in the air. How much work is done on the ballerina during the lift? GIVEN: m = 40 kg d = 1.4 m - during W = ? WORK: F = m·a F =(40 kg)(9.8 m/s 2 ) = 392 N W = F·d W = (392 N)(1.4 m) W = 549 J during lift F W d

24. ♦Power is the amount of work done in an amount of time. Power

25. P = Power (Watts - W) t = s W = Work (J)

26. Power GIVEN: F = 30 N d = 10 m t = 5 s P = ? WORK: P = F  t P = (30 N) (10 m)  P = 60 W A cart is pushed by a 30 N force against friction for a distance of 10 m in 5 seconds. Determine the power needed to move the cart. P Fd t

27. Power GIVEN: W = 2289 J P = 1730 W t = ? WORK: t = W  P  1730 W t = 1.32 s u Jerome does 2289 J of work while running up a flight of stairs. If his power is 1730 W, how long does it take him to climb the stairs? P W t

28. 3 Ways to Describe How Well Machines do Work 1)Actual Mechanical Advantage 2)Ideal Mechanical Advantage 3)Efficiency (the most common)

29. Two forces are involved when a machine is used to do work. Input and Output Forces 1) The force that is applied to the machine is called the input force, also called the effort force. 2) The force applied by the machine is called the output force, also called the resistance force.

30. Actual Mechanical Advantage F R = resistance force F E = effort force The ratio of the resistance force (output force) to the effort force (input force) is the actual mechanical advantage (AMA) of a machine.

31. Ideal Mechanical Advantage d E = effort distance d R = resistance distance The mechanical advantage of a machine without friction is called the ideal mechanical advantage, or IMA.

32. Efficiency is a measure of how much of the work put into a machine is changed into useful output work by the machine. Efficiency

33. Calculating Efficiency Efficiency is usually expressed as a percentage by this equation:

34. Machines can be made more efficient by reducing friction. This usually is done by adding a lubricant, such as oil or grease, to surfaces that rub together. Increasing Efficiency

35. You are using a lever to lift the edge of a crate in order to slide a roller under it. The crate weighs 5250 N. You are able to exert a force of 400 N and move the handle of the lever 1.20 meters. The crate is lifted a distance of 0.08 meters. (a) What is the input work done? Machines and Work – Problem #1 W=Fd=400 N x 1.2 m (a) W in = 480 J (b) What is the output work done? W=Fd= 5250 N x 0.08 m (b) W out = 420 J

36. You are using a lever to lift the edge of a crate in order to slide a roller under it. The crate weighs 5250 N. You are able to exert a force of 400 N and move the handle of the lever 1.20 meters. The crates is lifted a distance of 0.0800 meters. (c) What is the efficiency of the lever? Efficiency = Work out /Work in = 420 J / 480 J (c) eff = 87.5% Machines and Work – Problem #1

37. You are using a lever to lift the edge of a crate in order to slide a roller under it. The crate weighs 5250 N. You are able to exert a force of 400 N and move the handle of the lever 1.20 meters. The crate is lifted a distance of 0.0800 meters. (d) What is the actual mechanical advantage of the lever? (d) AMA = 13.1 (e) What is the ideal mechanical advantage? (e) IMA = 15 Machines and Work – Problem #1

38. A ramp is used to raise barrels that weigh 824 N up onto a 1.50 meter high loading dock. The ramp is 4.00 m long. The ramp’s actual mechanical advantage is 2.20. (a) How much effort must be exerted to roll the barrels up the ramp? Machines and Work – Problem #2 (a) F E = 375 N (b) What is the input work? (b) W in = 1500 J

39. A ramp is used to raise barrels that weigh 824 N up onto a 1.50 meter high loading dock. The ramp is 4.00 m long. The ramp’s actual mechanical advantage is 2.20. (c) What is the output work? Machines and Work – Problem #2 (c) W out = 1236 J (d) What is the efficiency of the lever? (d) eff = 82.4%

40. A ramp is used to raise barrels that weigh 824 N up onto a 1.50 meter high loading dock. The ramp is 4.00 m long. The ramp’s actual mechanical advantage is 2.20. (e) What is the incline plane’s ideal mechanical advantage? (e) IMA = 2.67 Machines and Work – Problem #2

41. Types of Simple Machines

42. A simple machine is a machine that does work with only one movement of the machine. There are six types of simple machines: lever, pulley, wheel and axle, inclined plane, screw and wedge. All of these can be classified as either a 1) Lever 2) Inclined Plan

43. Levers A lever is a bar that is free to pivot or turn around a fixed point. The fixed point the lever pivots on is called the fulcrum.

44. A pulley is a grooved wheel with a rope, chain, or cable running along the groove. This is a type of lever. Levers

45. A wheel and axle is a simple machine consisting of a shaft or axle attached to the center of a larger wheel, so that the wheel and axle rotate together. This is a type of lever.

46. Inclined Planes A sloping surface, such as a ramp, that reduces the amount of force required to do work, is an inclined plane.

47. Inclined Planes A screw is an inclined plane wrapped in a spiral around a cylindrical post. You apply the input force by turning the screw. The output force is exerted along the threads of the screw.

48. Inclined Planes The wedge is also a simple machine where the inclined plane moves through an object or material.

49. Simple Machine Mini-project Select a Simple Machine Write the definition Draw a Picture of it Find the earliest known use of it List 3 common uses today How does it make our work easier? (Use Chp. 12 in your book or a smart phone to help) Turn this in before your leave