2.  Work and Power

3.  Work is done on an object when the object moves in the same direction in which the force is exerted. Ex: pushing a child on a swing Ex: pulling books out of a bag

4.  In order for work to be done, your object MUST move some distance in the same direction of your force.

5. Is Work Being Done?  Is work being done on the barbell?  Work IS being done. How do you know? The barbell is moving in the same direction of the force (up).

6. Is Work Being Done?  Is work being done on the cart?  Work IS being done. How do you know? The cart is moving in the same direction of the force (to the left).

7. Is Work Being Done?  Is work being done on the rug?  Work IS NOT being done. How do you know? The rug is not moving in the same direction of the force (force is up, rug is moving to the left).

8. Power  An object that has more power than another object can do more work in the same amount of time.  Power is the rate at which work is done.

9. Calculating Work and Power Formula Units Work(N  m) or Joules Power Watts Work and Power are both SCALARS. Direction is NOT included.

10. Example  Ex: Jessie applies 20 N to a box and moves it 15 m in 5 s. What was the work done on the box? How much power does Jessie have?

11. Calculating Work  W = F  d  W = 20  15  W = 300 J

12. Calculating Power

13. A bit more on energy & work

14. ENVIRONMENT Diagram adapted from pg 133 Five Easy Lessons by Randall D Knight Energy transfer Work = W SYSTEM Motion energy = “ kinetic energy ” = KE Stored energy = “ potential energy ” = PE Mechanical energy = ME = KE + PE System boundary

15. Kinetic Energy  This is motion energy.  Like “ Fast Cash ”, $ in your pocket, it is going to be spent.  KE = ½ mv 2

16. Potential Energy  Stored Energy – due to position, location, or chemicals.  “ Stored ”, $Cash in bank, Savings Accounts  PE grav = mgh  PE spring = ½ kx 2

17. Energy is like Money  Money, like energy, has no single unique definition.  A simple, yet inadequate definition of energy as “ the ability to do work ” is analogous to a definition of money as “ the ability to purchase goods ”  Energy, as a quantity can be either transferred to or from the system or transformed within the system. Adapted from Five Easy Lessons by Randall D Knight

18. Money can be transformed in various ways.  Dollars can be transformed to coins or vice versa.  Yen can be transformed to euro.  I can transform some of my money into stocks or bonds.

19. Energy can be transformed in various ways.  I can transform some of the chemical energy stored in my body into kinetic energy by moving my body.  A wind farm can transform kinetic energy of air molecules into electrical energy.

20. Money can transferred in various ways. By working here at NNHS, I get money automatically transferred from the City of Newton into my savings account at the bank. Then, every week, I transfer that money to gas stations, grocery stores, Target, day care, etc. etc.

21. Energy can transferred in various ways.  Energy can be transferred from one object to another. A stretched bowstring (Elastic Potential Energy) can launch an arrow. (Kinetic Energy)  A moving cue ball (Kinetic Energy) hits a stationary billiard ball. The cue ball stops, and the billiard ball moves (Kinetic Energy).

22. Again...  Energy can be transferred to or from a system. OR  Energy can be transformed within a system.

23. Work : energy in transfer  Work is only present when energy is being transferred to or from a system  Work = Force  Distance  Work can be (+) positive or (-) negative.  + Work : earn $$ at a job = $$ coming in  - Work : pay bills = $$ going out

24. Units are Joules  The units for both work and energy are Joules. (Hint: think “ jewels ”, as in you earn “ jewels ” in the money analogy)

25. Energy is conserved.  Like momentum, the total energy in an isolated system is conserved.  Be careful about whether your system is truly closed or not.

26. Energy Graveyard R.I.P You can ’ t get it back... Disapated, but not forgotten. Energy appears to be “ lost ”, when it is transferred to it ’ s surroundings in the form of heat due to friction, or sound, or vibrations... etc. etc.