1. Work and Power Physics Mrs. Coyle

2. What is the direction of the component of the force that causes the sled to move?

3. What component of F applied causes this object to move?

4. Work The work done on an object by an external constant force: W= F d cos   is the angle between the displacement d and the force F.

5. Work Is a scalar quantity. Unit: Joule= Newton ∙ meter J= N∙m James Prescott Joule English, 19thCentury

6. When the force is in the direction of the displacement,  =0 o : cos 0= 1 => W= F d and work is positive Which force acting on the sled does positive work?

7. When the force acts opposite to the direction of the displacement,  =180 o : cos 180   = -1 => W= F d and work is negative Which force acting on the sled does negative work?

8. When the force acts perpendicular to the displacement, then the work done by that force is zero. What force(s) acting on the sled does no work?

9. Question Does the centripetal force do work?

10. Meaning of Sign of Work When the work done by an external force onto the system is: + : the system gains energy - : the system loses energy

11. Example 1 How much work was done by the 30N applied tension force to move the 6kg box along the floor by 20m? (A: 300J) How much work did the weight do in the same case? If µ=0.2, how much work did the friction do? (Hint: Use F xnet to calculate N) (A: -136J)

12. Work done by Resultant W= W 1 + W 2 + W n Since work is a scalar, the net work done by a resultant force is equal to the sum of the individual works done by each individual force acting on the object.

13. Example 2 How much total work is done on a sled that moves at constant velocity?

14. Work is the area under an F vs d graph.

15. Power, P= W = Fdcos  t t Rate Scalar Unit : J/s = W ( Watt) James Watt (18 th Century) developed the steam engine.

16. Power P= W = Fdcos  Fvcos  t t

17. kiloWatt∙ hour Does a kWatthour (kWh) measure: Power or Work

18. Pile Driver

19. Example 3 A pile driver drives a pile in 10 m with a force of 2000N in 0.05s. 1.Calculate the work done by the driver. (A: 2 x10 4 J) 2.Calculate the power of the driver. (A: 4x10 5 W)

20. Example 4 An engine moves a boat through the water at a speed of 12m/s. The force exerted by the engine is 5500N. What is the power of the engine? Answer: 6.6 x 10 4 W