Chapter 6

2. Chapter 6 Work, Energy, and Power

3. Introduction • Universe is made up of matter and energy. • Energy is the mover of matter. • It has several forms. To understand this concept we will begin with a closely related physical concept.

4. WORK For motion in a straight line the WORK done by a force is defined as the product of the component of the force in the direction of motion times the distance moved.

5. q x

6. Work is a scalar quantity. • Work can be negative. • Work is the transfer of energy from one entity to another by way of the action of a force applied over a distance. The point of application of the force must move if work is to be done. • Pushing on a wall and wall doesn’t move (no work done on the wall)

7. The Units of Work • N.m {Joules (J)} or ft.lb • 1 erg = 10-7 J. • 1 ft.lb = 1.355 J. • 1 BTU = 778 ft.lb (energy of one wooden kitchen match)

8. ENERGY • Energy is a measure of the change imparted to a system??? • It can be mechanically transferred to an object when a force does work on that object. • Further, when an object does work, it gives up an amount of energy equal to the work it does.

9. MECHANICAL ENERGY • When work is done on an object, the object generally has acquired the ability to do work. • This is called energy and it has the same units as work. Two Types of Mechanical Energy Kinetic Energy Potential Energy

10. Kinetic Energy • It is the energy possessed by an object because of its motion. • It is a square law. • Total Work (work done by all forces acting on mass m) = DKE

11. Potential Energy • Energy of position or configuration • Demo – Dart Gun • Other examples - Springs, bow, sling shot, chemical energy, and gravitational potential energy • The latter is PEG = mgh

12. Gravitational Potential Energy • The potential energy of an object depends on a reference position. • It represents the work done against gravity to put the mass m in its position h above some reference position. • It is an energy of position.

13. Note Work to Stop KE 0

14. The Work-Energy Theorem The net work done on an object is equal to the change in the kinetic energy of the object. Net Work = DKE From text: when work is done on a point mass or a rigid body, and there is no change in PE, the energy imparted can only appear as KE. Insofar as a body is not totally rigid, however, energy can be transferred to its parts and the work done on it will not precisely equal its change in KE.

15. CONSERVATION OF ENERGY • Energy cannot be created or destroyed. • It may be transformed from one form into another, but the total amount of energy never changes. • Energy lost due to friction is actually not a loss; it is just a conversion. • Energy Conservation in Satellite Motion (Next slide)

16. Perigees Hyperbola Parabola Circle Ellipse Ellipse Energy is conserved along all of these paths. Apogees

17. Condition for Conservation of Mechanical Energy • No work can be done on the object by a nonconservative force. • A nonconservative force is a force that converts mechanical energy into another form. • Example: Friction

18. No work is required to maintain circular motion at constant speed.

20. POWER or

21. W 550 ft.lb /s = 1 hp Units - J/s = 1 hp = 746 J/s = 746 W 1 BTU/hr = 0.293 W 100 W bulb = 0.1341 hp 250 hp engine = 186,450 W

22. The Kilowatt-Hour • The kilowatt-hour is a unit of energy. • If a force is doing work at a rate of 1 kilowatt (which is 1000J/s), then in 1hour it will do 1 kWh of work. • 1 kWh = 3.6 x 106 J = 3.6 MJ

23. Machines • If no losses then work input = work output (F.d)input = (F.d)output • Examples - levers, block and tackle, etc.

24. F D = F D D D

25. EFFICIENCY • Efficiency = work done/energy used • Useful energy becomes wasted energy with inefficiency. • Heat is the graveyard of useful energy. • EER = energy efficiency ratio It is the output capacity (BTU/hr)/input energy (Watts) (Output capacity represents energy moved.)