Ch 4: Work, Energy and Power

1. Ch 4: Work, Energy and Power

2. Chapter 4 – Work, Energy and Power Work: Work is done on an object whenever a force is exerted on an object through some distance. If the force is constant and parallel to the direction of movement then: W = F d W = 0, if d = 0 No work done if object doesn’t move. W = 0, if F = 0 No work done, whenever there is no force.

3. Chapter 4 – Work, Energy and Power Work: Work is done on an object whenever a force is exerted on an object through some distance. If the force is constant and parallel to the direction of movement then: W = F d W = 0, if F is perpendicular to d

4. Chapter 4 – Work, Energy and Power Examples: A person pushes with a 40 lb force for a distance of 100 ft. How much work was done? W = F d W = ( 40 lb ) ( 100 ft ) W = 4000 ft lb

5. Chapter 4 – Work, Energy and Power Examples: A person pushes with a 110 N force for a distance of 30 m. How much work was done? W = F d W = ( 110 N ) ( 30 m ) W = 3300 N m or W = 3300 J 1 Joule (J) = 1 N m

6. Chapter 4 – Work, Energy and Power Energy: Energy is something that an object possesses. The amount of energy that an object contains, is a measure of how much work it is capable of doing. Energy can be thought of as stored work (and it has the same units as work). There are many types of energy: Kinetic Energy: A moving object is capable of doing work due to it’s motion. This amount of work is called the object’s kinetic energy and it has a specific size: 1 2 E = mv K 2

7. Chapter 4 – Work, Energy and Power Example: What is the KE of 100 kg of water moving at 1.2 m/sec? 1 2 E = mv K 2 1 2 E = ( 100 kg ) ( 1.2 m/s ) K 2 1 2 2 E = ( 100 kg ) ( 1.44 m /s ) K 2 2 2 E = 72 ( kg m )/s K E = 72 J K

8. Chapter 4 – Work, Energy and Power Types of energy: Potential Energy: An object may be capable of doing work by virtue of its position relative to other objects. E = m g h P

9. Chapter 4 – Work, Energy and Power Energy Equivalents: 130,000,000 J = ... Chemical energy in one gallon of gasoline. Chemical energy in 65 ‘Big Macs’. Electrical energy consumed by a 200 watt color TV in 7 days of constant operation. Light energy in about 10 m of sunlight over 4 hrs. 2 Kinetic energy of a 2000 kg car going 800 mi/hr, or enough for same car to repeat 0 to 80 mi/hr 100 times. Nuclear energy released in fission of 2 milligrams of Uranium 235.

10. Chapter 4 – Work, Energy and Power 1 2 Kinetic Energy Fact: E = mv K 2 The kinetic energy increases dramatically with increasing speed. If ‘v’ doubles, then KE will increase by 4 times. If ‘v’ triples, then KE will increase by 9 times. If ‘v’ quadruples, then KE will increase by 16 times.

11. Chapter 4 – Work, Energy and Power 1 2 Kinetic Energy Fact: E = mv K 2 The kinetic energy increases dramatically with increasing speed. 4 x speed 35 mi/hr 70 mi/hr 140 mi/hr 16 x energy

12. Chapter 4 – Work, Energy and Power 1 2 Kinetic Energy Fact: E = mv K 2 The energy content of air is the same as water. KEair = KEwater 1 1 2 2 m v = m v 2 2 2 2 m v = m v 2 2 (1.3 kg) va = (1000 kg) vw 2 2 va = (830) vw va = (29) vw

13. Chapter 4 – Work, Energy and Power 1 2 Kinetic Energy Fact: E = mv K 2 The energy content of 29 mi/hr air is the same as 1 mi/hr water. 29 mi/hr air = 1 mi/hr water 58 mi/hr air = 2 mi/hr water 87 mi/hr air = 3 mi/hr water . . . 145 mi/hr air = 5 mi/hr water . . . 290 mi/hr air = 10 mi/hr water

14. Chapter 4 – Work, Energy and Power Power: Power is the rate at which work is being done [or also the rate at which energy is consumed]. W E P = or P = t t

15. Chapter 4 – Work, Energy and Power Examples: The minimum work required to raise a 800 N person up 10 m, is: W = F d W = (800 N) (10 m) = 8000 J If this work is done in 60 sec, then what is the power? W 8000 J J P = = = 133 = 133 watts t 60 sec sec or 1 hp 133 watts = 0.18 hp ( ~1/6 hp ) 746 watt

16. Chapter 4 – Work, Energy and Power Examples: A ‘Big Mac’ contains about 2,000,000 J of chemical energy. If all this energy could be used to power a 60 watt light bulb, how long could it run? E P = t E 2,000,000 J t = = P 60 watt J t = 33,000 J/sec t = 33,000 sec ( ~ 9 hr )

17. Chapter 4 – Work, Energy and Power Examples: How much work is required to lift a 200 N weight a distance of 0.5 m, a total of 40 times. 1 time: W = F d = (200 N) (0.5 m) = 100 J 40 times: W40 = 40 x W1 = (40) (100 J) = 4000 J This can be converted to food calories: 1 Calorie 4000 J = 0.95 Calorie 4200 J

18. Chapter 4 – Work, Energy and Power Examples: How much work is required to lift a 200 N weight a distance of 0.5 m, a total of 40 times. This can be converted to food calories: 1 Calorie 4000 J = 0.95 Calorie 4200 J However, the human body is only about 4% efficient, so to do 1 Cal. of work uses about 25 Cal. of food.

19. Chapter 4 – Work, Energy and Power Conservation Laws General Format: The total amount of in a closed system, never changes. Quantities that can fill the blank: Mass 10 kg 10 kg

20. Chapter 4 – Work, Energy and Power Conservation Laws Quantities that can fill the blank: Mass Energy

21. Chapter 4 – Work, Energy and Power Conservation Laws Quantities that can fill the blank: Mass Energy Electric Charge Linear Momentum Angular Momentum (the amount of rotational/spin motion in a system)