Energy & Work

1. Energy & Work

2. Energy • What is energy? • The capacity to do work • What is Work? • The transfer of energy from one physical system to another Bottom line: it is the outward manifestation of energy which causes a change in something (e.g. changes in motion or position).

3. Energy is measured in Joules • Joules = newton*meter = kg*m2/s2 • In English units we call them calories or kilocalories

4. Types of energy • Kinetic energy – energy of motion • Potential energy – stored energy

5. Kinetic Energy • Mechanical energy – energy inherent in objects in motion • Light energy – highly organized packets of energy called photons • Electrical energy – energy generated by charged particles • Heat energy – atomic and molecular motion create heat

6. Potential Energy • Positional energy – energy stored due to position of object relative to some force • Electrical energy – stored charged particles • Bond energy – energy stored in the chemical bonds holding 2 or more atoms together

7. Laws of thermodynamics • laws of the movement of heat?...why heat? • Push your book of the desk! • 1) Positional energy to simple kinetic energy • 2) positional energy also converted to heat due to friction • 3) book hits ground, the kinetic energy is converted to heat energy in the book and the ground • 4) pick it back up and bond energy is converted into mechanical energy in my muscles • 5) both bond and mechanical muscle energy are converted to heat • 6) the heat is eventually radiated into the atmosphere

8. Laws of thermodynamics • 1st Law - Conservation of Energy • Energy is neither created nor destroyed • Energy within a system is only converted from one form of energy to another • Energy can be moved into or out of a system • Total energy in the universe is a constant

9. Laws of thermodynamics • 2nd Law – Entropy • Energy naturally flows from more ordered forms of energy to less ordered forms of energy • Corollaries: • The total disorder in any closed system always increases. • Every conversion of energy from one form to another converts some energy into disordered energy in the form of heat.

10. Back to Work • If work is the transfer of energy and energy can be neither created or destroyed…….the work done within a closed system…….. • Decreases over time? • Remains constant ?

11. How is Work measured? • The force applied to an object times the distance the object moves due to that force. • W = F*d (Recall, F=ma) • Peculiarities: • 1) Movement perpendicular to the direction of the force does not result in work • 2) Supporting an item does not result in work. If the distance moved is zero, then there is no work.

12. Animal Movement • Moving a body is doing work • There are a variety of ways of dealing with forces and distance that relate to the ease of accomplishment of work • Humans, being animals, have made a habit of using these ways to accomplish work more easily in the form of Simple Machines

13. A machine is a device that helps make work easier to perform • Transferring a force from one place to another • Changing the direction of the force • Increasing the magnitude of a force • Increasing the distance or speed a force is applied

14. 4 Basic Simple Machines • Levers • Inclined planes • Pulleys • Wheel and axle

15. Incline Plane Horizontal distance Vertical distance

16. 50kg How much force is required to lift a 50kg weight 3 meter off of the ground? F = m a F = 50 kg (9.8 m/s2) = 490N 3 m

17. 50kg How much work vs. gravity needs to be done to move a 50 kg weight up 3 meters? Work = F*d = 490N*3m = 1470 Joules 3 m 5 m

18. 50kg How much force do you need to apply to move the same weight up the ramp? Work = F*d F = W/d = 1470J/5m = 294 N 3 m 5 m

19. 490 N = 1.66 294 N Force w/o = Force w Length of plane 5 m = Height of plane 3 m How much easier is it to do the work (1470J) with the inclined plane than without? Mechanical advantage = force without incline / force with incline For inclined planes you get the same number by: = 1.66 3 m 5 m

20. Mechanical Advantage • force produced by the machine divided by force applied to the machine • F out / F in

21. Resistance force Effort Force d e Levers

22. Resistance force Effort Force d e d r

23. R E F out F in d e What is the mechanical advantage of the lever system? 5 kg 15 kg 15 kg * 9.8 m / s2 = 5 kg * 9.8 m / s2

24. R E d e If the mechanical advantage is 3 :1 can you calculate dr and de? 5 kg 15 kg If you need to move the Fr 9 meters what is the distance the Fe will have to move?

25. F r F e This is a different type of lever; what is different from the previous lever?

26. F r F e Where are dr and de? de dr

27. F r F e This is the last type of lever; what is different?

28. F r F e Where are dr and de? dr de