1. Applied Physics Chap 4 Energy1 Chapter 8: Energy The universe is composed of two substances called matter and energy which are interrelated on some fundamental level (Einstein’s law E = mc 2 ) But: we really don’t know what Energy is.

2. Applied Physics Chap 4 Energy2 Video: Introduction to energy and work

3. Applied Physics Chap 4 Energy3  matter possesses Inertia and Gravity  matter forms all the stuff that makes up our world.  Matter is measured by its mass in kg.  Energy appears in a variety of different forms.  Energy can be changed from one form into another  Energy causes changes in matter What we do know about matter: What we do know about energy:

4. Applied Physics Chap 4 Energy4 Energy: The ability to cause a change in matter. Different types of Energy: Thermal Energy: Energy from heat and fire Mechanical Energy: energy from motion or position Chemical Energy: Burning fuel, food, batteries Nuclear Energy: Energy from the atom Electrical Energy: Energy from electrical interactions Radiant Energy: Energy in the form of light, etc. Work: the transfer of energy from one kind to another through motion

5. Applied Physics Chap 4 Energy5 Video: energy exchanges

6. Applied Physics Chap 4 Energy6 Video: Physical energy exchanges

7. Applied Physics Chap 4 Energy7 Mechanical energy: is energy that results from the position of something (called Potential Energy) or from its motion (called Kinetic Energy). Potential Energy PE stored energy Gravitational PE: is energy stored in an objects height above the ground Chemical PE: is energy stored in the position of atoms in a molecule Elastic PE: is energy stored in a spring or rubber band Kinetic Energy: KE, energy in motion.

8. Applied Physics Chap 4 Energy8 Video: Kinetic and Potential Energy

9. Applied Physics Chap 4 Energy9 Gravitational Potential Energy depends upon: the mass of the object being lifted (in kg). the height the object is lifted to (in meters). PE g = Gravitational Force (weight) x Change in height. PE = F g  h Since Fg = mg then PE g also = mg h h m F m

10. Applied Physics Chap 4 Energy10 Units of Energy: Joule 1 N  1 m = 1 Joule 1 Newton pushing something a distance of 1 meter performs 1 Joule of work James Joule: 1818 - 1889 The English physicist James Prescott Joule (1818-1889) proved that mechanical and thermal energies are inter-convertible on a fixed basis, and thus he established the great principle of conservation of energy.

11. Applied Physics Chap 4 Energy11  The product of the force on an object and how far the object is moved by that force. Wk = Force  distance  Work measures how much energy is converted from one form into another Since work is equal to the quantity of energy transferred, then the units for work would also be JOULES. WORK: The transfer of energy through motion.

12. Applied Physics Chap 4 Energy12 Video: energy and work

13. Applied Physics Chap 4 Energy13 WORK DONE AGAINST FRICTION: F m F m d Calculating work done against friction: Work = Force of Friction x distance moved. Wk = F f  d Work is done by changing chemical energy to kinetic energy to thermal energy which is lost to the surrounding air.

14. Applied Physics Chap 4 Energy14 Work done lifting a mass to a height.  h F As the object is lifted, work is done converting chemical energy (in muscles) into gravitational potential Energy which is stored in its position. on the ground, the box has 0 J of PEg After it is lifted the box has PE g = mgh Since mg = Weight then: PE g = W  h

15. Applied Physics Chap 4 Energy15 The faster something is moving, the more kinetic energy it contains. KE = ½ mv 2 KE depends on: The square of the object’s velocity in m/s The objects mass in kg. KINETIC ENERGY Mechanical energy that comes from the motion of an object.

16. Applied Physics Chap 4 Energy16 Recall: work is done when energy is changed from one form to another through motion. M = 1500 kg V = 15 m/s Work is done accelerating an automobile because, energy stored in gasoline is changed to KE by burning it in the engine. The amount of work done is equal to the KE of the car after it reaches a top speed.

17. Applied Physics Chap 4 Energy17 Kinetic energy and the automobile M = 1500 kg V = 15 m/s M = 1500 kg V = 30 m/s Since all the KE comes from gasoline, it takes 4 times as much gasoline to travel at 30 m/s as at 15 m/s. Double the speed Increase KE by 4 times as much.

18. Applied Physics Chap 4 Energy18 Power: is the rate that work is done To increase power, just do the same amount of work in a shorter time. Similarly, taking longer to do work requires less power. power is given the units of Watts. Named after James Watt the inventor of the steam engine.

19. Applied Physics Chap 4 Energy19 Law of conservation of energy. Energy cannot be created or destroyed. It can only be transformed from one form into another, leaving the total amount of energy unchanged. Law of conservation of Mechanical energy: The sum total of KE + PE in a system does not change. but at any point energy is either PE  KE ME = Constant KE i + PE i = KE f + PE f

20. Applied Physics Chap 4 Energy20 Conservation of energy in a Pendulum. B As the weight falls toward point B. PE is converted into KE until at Point B all the energy is Kinetic. When Pendulum is released from point A, it has PE but no KE. A As the weight rises toward point C. KE is converted back into PE until it stops moving at point C. C

21. Applied Physics Chap 4 Energy21 Height = 10 m At the top KE = 0 J PE = mgh = (5 kg)(9.8)(10m) = 490J ME = KE + PE = 0 + 490J = 490 J At the bottom PE = 0 J KE = 490J ME = KE + PE = 0 + 490J = 490 J Conservation of Mechanical energy Consider a heavy rock at the top of a cliff

22. Applied Physics Chap 4 Energy22 Conservation of Energy and the Roller coaster No PE No KE Work is done increasing the car’s PE PE changes to KE

23. Applied Physics Chap 4 Energy23 Conservation of energy: The Pile Driver. A.) Work is done lifting the weight, converting chemical energy into potential energy. B.) When the weight falls it PE is converted into KE C.) When the weight hits the piling it does work driving the piling into the ground. Energy is converted from KE to thermal energy through friction with the ground.

24. Applied Physics Chap 4 Energy24 Work, Machines and Mechanical Advantage For example, to accomplish work Wk = F  d we could use a large force to push an object a short F  d or a small force times a long distance F  d as long as the product of force and distance equal the same amount of work We use machines because they permit us to do the same amount of work while using a smaller amount of force at the expense of a longer effort distance. Wk = F  d = F  d

25. Applied Physics Chap 4 Energy25 We need to lift a 50N box to a height of 2.0 m. The work we need to do is: Wk = F  d = 50N  2m = 100 J F 2 m 50N However its easier to use a ramp Using a ramp does the same work with less force. 2 m 50 N We could lift it straight up: The force we would need:

26. Applied Physics Chap 4 Energy26 Mechanical Advantage: MA: The ratio of the output force of a machine to the input force you have to exert. Effort Force F is the force a worker has to apply to lift an object Resistance force R is either the weight of the object or the force needed to move the object. Mechanical advantage is a multiplier. You multiply the effort force by the MA to find how much weight can be lifted.

27. Applied Physics Chap 4 Energy27 Simple Machines: Simple machines devices that use the principle of mechanical advantage to permit accomplishing a certain amount of mechanical work with less force. There are six basic kinds of simple machines: Levers, pulleys, wheel and axel, screw, wedge and inclined plane. In combination, many other types of compound machines can be constructed.

28. Applied Physics Chap 4 Energy28 Levers: a device composed of a rigid arm called the lever arm, and a rest point called a fulcrum. Fulcrum Input Force F Resistance force R Effort Length E Resistance Length L

29. Applied Physics Chap 4 Energy29 Fulcrum: point around which the lever rotates. Effort Length E : the distance from the point where you apply force to the fulcrum. Resistance Length L. The distance from the point where the output force is applied to the fulcrum. Effort force: F The amount of force that the worker or the machine has to apply to do the work Resistance force: R Either the weight of the object or the force that must actually be used

30. Applied Physics Chap 4 Energy30 Mechanical advantage for a lever. Mechanical advantage is equal to either: The ratio of effort length to resistance length OR The ratio of resistance force to effort force.

31. Applied Physics Chap 4 Energy31 Screw: An inclined plane wrapped around a cylinder. A screw is simple a long inclined plane that is rotated to continue moving an object upward. When coupled with a wheel and axel like a screw driver, it allows a large quantity of force to be applied in lifting an object up or down. Wedge: A double inclined plane that is used to turn a downward force sideways. Example: a log splitting wedge takes the downward force from the maul and redirects it at a 900 angle in each direction to force the log apart.

32. Applied Physics Chap 4 Energy32 Resistance Length L Effort Length E Wheel and Axel: a special kind of lever. Effort Length Resistance Length

33. Applied Physics Chap 4 Energy33 Effort length Resistance length Example: Resistance Length = 10 cm and Effort Length = 20 cm. Wheel and Pulley

34. Applied Physics Chap 4 Energy34 Gears are determined like unequal diameter pulley’s as a ratio either of the radius of the gears or as a ratio of the number of teeth on each gear. A

35. Applied Physics Chap 4 Energy35 Pulley systems: a special type of wheel and axel

36. Applied Physics Chap 4 Energy36 300 N Effort force = 300 N MA = 1 300 N Effort force = 150 N MA = 2 300 N Effort force = 100 N MA = 3 Mechanical advantage in pulleys.