Grantham University, GS103

Homework: Chapter 3

1. What forms of energy are used in the following sports: a)Surfing Kinetic energy of the waves is passed on to the surfboard.

b)Nascar racingChemical energy of the gasoline (fuel) which ignites the cylinder mixture of air and gasoline.

c. Hang Gliding ANSWER Potential energy initially and then a mixture of wind energy and potential energy.d. SkiingANSWER Skiing first uses potential energy as the skier sets off. The potential energy of the skier while descending changes to kinetic energy and at the extreme point of descent, kinetic energy changes to zero and potential energy also becomes nil.e. GolfMechanical energy of the golfer, aided by a small bit of wind energy, which makes the ball wobble in the air.

f. Mountain climbingMechanical energy of the climber.

2. Would it be energy efficient to use a solar water heating system in Alaska? .Why or why not? What would be a more efficient energy choice to heat water in cold climates?Answer: It is a myth that Alaska is colder than many places in the world. In fact, in summertime, temperatures in Alaska have been known to reach 90oF. In the winter, temperatures in Alaska are low. Solar water heating in Alaska may not be very efficient in the winter. Let us analyze why energy is required to heat water. Water turns vapor at 100oC. Say we have water at 100C. To heat the water there must be some kind of energy feed to the water. This heat energy has the unit Joules or KJ, If we assume that water required should be at 700C, then for the same mass of water under identical conditions, KJ required will depend upon the initial temperature of the water. The lower the temperature below 70 deg Celsius, the more KJ is required to attain the final temperature, or more energy will be required.

This will depend upon the conditions as to what is available and what is not. Solar, Wind, Geothermal, Wood and kindling, natural gas all of these may be turn out to be the most economic. Example if say drilling for oil is going on and the gas (which is found) before petroleum is burned off. Then, natural gas may be cheaper. However, if a forest is located nearby and dry wood and kindling’s are available, then that can be

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used. For the engineer, the efficiency or the practical utility of a system depends upon not only the system parameters but also the cost.

3. Is it possible to use alternative energy sources to meet the current energy demands of the United States? What are some of the environmental costs for different forms of alternative energy sources? (Hint: Wind turbines kill birds and bats.)

4How do “warm-blooded” animals warm their blood? What form of energy do “cold-blooded” animals use to warm their blood and bodies?

“Warm blooded” animals need to eat regularly. The food they eat is transformed by chemical reactions, on being acted upon by enzymes etc. inside them, giving out energy which is used for warming the blood. “Cold Blooded” animals warm themselves and their blood by absorbing heat energy by conduction (resting with their bodies in contact with warm objects) or by basking in the sun’s radiated heat energy.

5. Does the total amount of energy in an isolated system change over time? Why or Why not?

In an isolated system, which is 100% efficient, the sum total amount of energy will remain the same. This is because of the law of conservation of energy. However, in actual practice, isolated systems with 100% efficiency do not exist as such a system is impossible

6. If you eat 500 calories per day (roughly one large order of fries) above your energy needs, how long will it take to gain 20 pounds? How long would you have to walk (assuming you burned 80 calories per mile walked) to burn off those 20 pounds?

The human body needs 3500 calories more than what is required to put on 1 lbs. Conversely, someone who wants to lose weight has to have an intake of 3500 calories per pound. Eating an excess of 500 calories/day will result in 3500 calories/week excess calorie intake. Hence, the body will gain 1 pound/week which gives the time required to gain 20 pounds as 20 weeks.Walking results in burning calories and you burn 80 calories/mileFor losing 20 pounds, miles you have to walk= (20*3500)/80= 875Therefore, a person will have to walk 875 miles to lose 20 pounds weight.

7. How much work against gravity do you perform when you walk up a flight of stairs five meters high (assuming that your body mass is 75 kilograms)? Compare this work to the energy done by a 100-watt light-bulb in an hour (60 minutes).

Height H

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5 meters

What data we have? Mass=75 kgsVertical distance attained= five mts.Before I start walking up the flight of spares, PE= zero. When I finish up the stairs,PE=mgh= 75Kgs *9.81ms-2 *5 mtsOr Potential energy=75*9.81*5=3678.75 Joules -------------------- (a) One 100 watt bulb does W work/hrThen Energy consumed by the bulb=0.1KwHr………………..(b)Also, 1 J= 2.778 x 10-7 kWhrGoing back to expression (a), 3678.75Joules= (3678.75*2.778 *10^-7)

This gives us , energy expanded in climbing the stairs =0.1021957x10-2

Or Em =0.1022/100=00.001022 kWh------------------------ ©Now on comparing (b) and (c), we come to know that the mechanical work we had done is only about 1% of the energy spent in keeping the 100 W bulb live for 1 hour.

8. Which has more gravitational potential energy: a 200-kilogram boulder 1 meter off of the ground, a 50-kilogram boulder 4-meters off the ground, or a 1-kilogram rock 200 meters off the ground? Which of these can do the most work if all of the potential energy was converted to kinetic energy?

In all these three cases, the gravitational potential energy is the same. This is because we will get the potential energy by the product of the mass and height, which is the same in all three cases. In the case of complete energy conversion from potential to kinetic, they will still have the same energy as energy cannot be created or destroyed i.e. here too the law of conservation of energy will apply.

9. Compared a car (mass of 2000 kg) moving at 10 miles per hour, how much kinetic energy (in Joules) does that same car have when it moves at 20 miles per hour? At 50 miles per hour? 75 miles per hour? (Hint: you will need to convert to miles per hour to meters per second).10 mph=Kinetic energy =M*(v2/2g); For 20 MPHAt 20 mph =2000X (20*o.447)/

We have the following scenarios1. At 10 mph, V1= 0.447*10=4.47 mts/sec

F=mg pulls the body downwards through its cg

Body c.g.

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2. 20 mph, velocity V2 =0.447X20=8.94m/s

3. At 50 mph, velocity V3=0.447 x 50 = 22.35 m/s

4. At 75 mph, velocity equals V4=0.447x75=33.52 m/s

One mile per hour = 0.447 meters per second.

KE=M*v2/2*9.81=101.94v2

Considering the above scenarios,

KE1 =101.94* (4.47)2 =2037 Joules

KE2 = 8147.41 Joules

KE3= 50921 Joules

KE4=114539 Joules

Since the car mass, the acceleration due to gravity are known, v can be given as

A function of KE or, KE=101.94 V2

Hence, the Kinetic energy will increase proportionately to the square of the velocity

10. Using your results from question 9, create a graph of those results. What does your graph suggest to you about the difficulty of stopping a car as its speed increases?*In MS Word, you can create a graph by going through Insert > Chart > x-y scatter and then inputting your velocity (m/s) and energy (J).

Kinetic EnergyIn Joules

Speed=meters/second