SPH3U UNIVERSITY PHYSICS

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1 /3/0 SPH3U UNIVERSITY PHYSICS ENERGY & SOCIETY L (P.36-4) We use mechanical enery (a combination of inetic enery and ravitational potential enery) to do mechanical wor every day. But mechanical enery is not the only type of enery. In fact, there are many types of enery in the universe, all of which involve inetic enery, potential enery, or both. NOTE! Different types of enery have different names. December 3, 0 3U3 - Conservation of Enery Electrical enery delivered to the stove heats the water in the pot. Thermal enery in the boilin water transfers to the pasta to coo it. December 3, 0 3U3 - Conservation of Enery

2 /3/0 The Sun emits radiant eneries, such as infrared radiation, visible, liht, and ultraviolet radiation. The Sun s enery comes from nuclear fusion reactions in its core. December 3, 0 3U3 - Conservation of Enery 3 At the hihest position above the trampoline, this athlete has the reatest amount of ravitational potential enery. The enery chanes to inetic enery as her downward velocity increases. The inetic enery then chanes into elastic potential enery in the trampoline to help her bounce bac up. December 3, 0 3U3 - Conservation of Enery 4 Chemical potential enery is released when firewors explode. Some of that enery is chaned into sound enery. December 3, 0 3U3 - Conservation of Enery 5

3 /3/0 Notice that heat is not listed as a form of enery. Heat is the transfer of thermal enery from a warmer body or reion to a cooler one. NOTE! In science, we use the word heat as a verb, not a noun. For example, the hot plate was used to heat the liquid to a temperature of 50EC and not, the hot water has more heat than the cold water. December 3, 0 3U3 - Conservation of Enery 6 ENERGY exists in many forms includin: C thermal C electrical C radiant C nuclear C ravitational C inetic C elastic C sound C chemical HEAT transfer of thermal enery from a warmer substance to a cooler one occurs until both substances are at the same temperature December 3, 0 3U3 - Conservation of Enery 7. Name at least one form of enery associated with each object in bold. (a) A bonfire roasts a marshmallow. (a) chemical, radiant December 3, 0 3U3 - Conservation of Enery 8 3

4 /3/0. Name at least one form of enery associated with each object in bold. (b) A baseball smashes a window. (b) sound, inetic December 3, 0 3U3 - Conservation of Enery 9. Name at least one form of enery associated with each object in bold. (c) A solar collector heats water for a swimmin pool. (c) radiant, nuclear December 3, 0 3U3 - Conservation of Enery 0. Name at least one form of enery associated with each object in bold. (d) A stretched rubber band is used to launch a projectile. (d) elastic, inetic December 3, 0 3U3 - Conservation of Enery 4

5 /3/0. Name at least one form of enery associated with each object in bold. (e) The siren of an ambulance warns of an emerency. (e) sound, electrical December 3, 0 3U3 - Conservation of Enery Enery Transformations The conversion of enery from one form to another is called an enery transformation. For example, in a microwave oven, electrical enery transforms into radiant enery (microwaves), which is then transformed into thermal enery in the food bein cooed. electrical enery û radiant enery û thermal enery ENERGY TRANSFORMATION the chane of enery from one form to another December 3, 0 3U3 - Conservation of Enery 3 Enery Transformations People often wonder how much enery there is in the universe and whether we will eventually run out of enery. Scientists have studied enery and enery transformations and have arrived at some important eneralizations. For example, they noticed that when one form of enery is transformed into another form (or forms) of enery, the quantity of one form is reduced by the same amount that the quantity of the other form (or forms) is increased. For example, a liht bulb may transform 00 J of electrical enery into 5 J of radiant enery and 95 J of thermal enery. However, the total amount of enery has not chaned. 00 J of electrical enery = 95 J of thermal enery + 5 J of radiant enery December 3, 0 3U3 - Conservation of Enery 4 5

6 /3/0 This eneralization, nown as the law of conservation of enery, is stated as follows: The total amount of enery in the universe is conserved. There is a certain total amount of enery in the universe, and this total never chanes. New enery cannot be created out of nothin, and existin enery cannot disappear; the enery that exists can only be chaned from one form into another. When an enery transformation occurs, no enery is lost. December 3, 0 3U3 - Conservation of Enery 5 LAW OF CONSERVATION OF ENERGY enery is neither created nor destroyed when enery chanes from one form to another no enery is lost START = ETOTAL DURING = E END NOTE! Evidence of the law of conservation of enery is all around us, but to notice it, you need to tae measurements and perform simple calculations. Consider 65.0 diver who performs a handstand dive from a 0.0 m hih divin platform into the water below. December 3, 0 3U3 - Conservation of Enery 6 Phase : Before the Dive The diver beins the dive in a handstand position on the platform of the divin tower. Since he is motionless, the diver s inetic enery is equal to zero (E =0), and his ravitational potential enery is calculated as follows: E = mh = (65.0 )(9.8 N/)(0.0m) E = 6370 J or 6.4 J December 3, 0 3U3 - Conservation of Enery 7 6

7 /3/0 Phase : Before the Dive At this point in the dive, the diver s total mechanical enery (E mech ) is equal to 6.4 J, the sum of his ravitational potential enery and his inetic enery: E E mech mech = E + E = 6.4 J + 0 J = 6.4 J December 3, 0 3U3 - Conservation of Enery 8 Phase : At the Halfway Point At the halfway point the diver is 5.0 m above the water s surface (and is still acceleratin at 9.8 m/s towards the water). At this point in the dive, the diver s ravitational potential may be calculated as follows: E = mh = (65.0 )(9.8 N/)(5.0m) E = 385 J or 3.J December 3, 0 3U3 - Conservation of Enery 9 Phase : At the Halfway Point At the halfway point, the diver s inetic enery may be calculated as well. But first his velocity at the halfway point must be calculated. v = v + ad v = (9.8m/s )(5.0m) v = m/s & since v = 0 E = mv E = 385 J = (65.0 )(9.899 m/s) or 3.J December 3, 0 3U3 - Conservation of Enery 0 7

8 /3/0 Phase : At the Halfway Point At the halfway point, the diver s total mechanical enery is still 6.4 J, the sum of his ravitational potential enery and his inetic enery: E E mech mech = E + E = 3.J + 3.J = 6.4 J December 3, 0 3U3 - Conservation of Enery Phase 3: At the Water s Surface When the diver reaches the surface of the water, his heiht above the water is 0 m and so his ravitational potential enery is equal to zero (E =0). His inetic enery is calculated as follows: v = v + ad v = (9.8m/s )(0.0m) v = 4.0 m/s & since v = 0 E = mv E = 6370 J = (65.0 )(4.0 m/s) or 6.4 J December 3, 0 3U3 - Conservation of Enery Phase 3: At the Water s Surface At the water s surface, the diver s total mechanical enery is still 6.4 J, the sum of his ravitational potential enery and his inetic enery: E E mech mech = E + E = 0J J = 6.4 J December 3, 0 3U3 - Conservation of Enery 3 8

9 /3/0 As you can see, while the diver s ravitational potential enery was transformed into inetic enery throuhout the dive, his total mechanical enery did not chane it was conserved. This fact can be used to help solve many problems. December 3, 0 3U3 - Conservation of Enery 4. A ball is dropped vertically from a heiht of.5 m; it bounces bac to a heiht of.3 m. Does this violate the law of conservation of enery? Explain. No, it does not violate the law of conservation of enery. This is because some of the inetic/elastic enery was transformed into other forms of enery such as sound/thermal. As a result, less enery was available to be transformed bac into inetic enery and thus ravitational potential enery. December 3, 0 3U3 - Conservation of Enery 5 3. A 56 diver jumps off the end of a 7.5 m platform with an initial horizontal speed of 3.6 m/s. (a) Determine the diver s total mechanical enery at the end of the platform relative to the surface of the water below. (a) E T@start = 4500 J December 3, 0 3U3 - Conservation of Enery 6 9

10 /3/0 3. A 56 diver jumps off the end of a 7.5 m platform with an initial horizontal speed of 3.6 m/s. (b) Apply the law of conservation of enery to determine the diver s speed at a heiht of.8 m above the water. (b) = 0 m/s December 3, 0 3U3 - Conservation of Enery 7 3. A 56 diver jumps off the end of a 7.5 m platform with an initial horizontal speed of 3.6 m/s. (c) Repeat (b) to find the maximum speed of the diver upon reachin the water. (c) = 3 m/s December 3, 0 3U3 - Conservation of Enery 8 4. A 0.0 ball is thrown straiht up from the ede of a 30.0 m tall buildin at a velocity of.0 m/s. The ball moves up to the maximum heiht and then falls to the round at the base of the buildin. Use the law of conservation of enery to answer the followin questions, assumin that the reference level for ravitational potential enery is round level (a) What is the total enery of the ball at the start when it had a velocity of.0 m/s? (a) E T@start = 0 J December 3, 0 3U3 - Conservation of Enery 9 0

11 /3/0 4. A 0.0 ball is thrown straiht up from the ede of a 30.0 m tall buildin at a velocity of.0 m/s. The ball moves up to the maximum heiht and then falls to the round at the base of the buildin. Use the law of conservation of enery to answer the followin questions, assumin that the reference level for ravitational potential enery is round level (b) What is the velocity of the ball at the maximum heiht? (b) = 0 (has stopped and is startin to come bac down) December 3, 0 3U3 - Conservation of Enery A 0.0 ball is thrown straiht up from the ede of a 30.0 m tall buildin at a velocity of.0 m/s. The ball moves up to the maximum heiht and then falls to the round at the base of the buildin. Use the law of conservation of enery to answer the followin questions, assumin that the reference level for ravitational potential enery is round level (c) What is the maximum heiht of the ball? (c) = 55 m December 3, 0 3U3 - Conservation of Enery 3 4. A 0.0 ball is thrown straiht up from the ede of a 30.0 m tall buildin at a velocity of.0 m/s. The ball moves up to the maximum heiht and then falls to the round at the base of the buildin. Use the law of conservation of enery to answer the followin questions, assumin that the reference level for ravitational potential enery is round level (d) What is the velocity of the ball when it hits the round? (d) = 33 m/s December 3, 0 3U3 - Conservation of Enery 3

12 /3/0 5. Many roller coasters have loops where carts roll on a trac that curves sharply up into the air. In the roller coaster shown, the cart must have a minimum speed of 0.0 m/s at the top of the loop to mae it around safely. Assumin that the roller coaster starts from rest at the top of the first hill and there is no friction on the roller coaster, what is the minimum heiht of the first hill needed to ensure success? h min = m December 3, 0 3U3 - Conservation of Enery 33 TEXTBOOK P.4 Q. U Chec Your Learnin December 3, 0 3U3 - Conservation of Enery 34