1. The concept of energy (and the conservation of energy) is one of the most important topics in physics. Work Kinetic energy Energy approach is simpler than Newton’s second law. Chapter 7: Work and Kinetic Energy-1 Reading assignment: Chapter 7.6-7.9 Homework: due Monday, Sept 26, 2005 Chapter 7: 30, 41, 43, 58, 65, 68

2. Work (as defined by a physicist) Definition: The work done on an object by an external force is - the ____________ of the component of the force in the direction of the displacement and the magnitude of the ________________.

3. How much work is done when just holding up an object?

4. How much work is done when the displacement is perpendicular to the force?

5. What is the work done by - the gravitational force - the normal force - the force F when the block is displaced along the horizontal. The total work is: - the sum of the work done by all forces - or: d = 10m  = 60° F = 10 N

6. What is the work done when lifting? - By the gravitational force? - By the applied force? Sign convention: W is positive: If F and d are parallel If energy is transferred into the system W is positive: If F and d are antiparallel If energy is transferred out of the system Strongest man lifting 140 kg boulder by 1 m.

7. Work is a ________ quantity. (not a vector) Work has units of newton·meter (N·m) = the joule (J)

8. A man loads a refrigerator onto a truck using a ramp. Ignore friction. He claims he would be doing less work if the length of the ramp would be longer. Is this true? Black board example 7.5

9. Black board example 7.1 A donkey is pulling a cart with a force of magnitude F = 500 N at an angle of 30º with the horizontal. Calculate the work done by the donkey as the cart is pulled for one mile (1648 m). 500 N

10. Work is the scalar product (or ____ product) of the force F and the displacement d. F and d are vectors W is a __________ quantity Definition of dot product and work

11. Scalar product between vector A and B Scalar product is ______________: Definition: _________________ law of multiplication:

12. Scalar Product using unit vectors: We have the vectors A and B: Then:

13. Black board example 7.2 A particle moving in the x-y plane undergoes a displacement d = (2.0i + 3.0j) m as a constant force F = (5.0i + 2.0j) N acts on the particle. Calculate (a)The magnitude of the displacement and the force. (b)The work done by F. (c)The angle between F and d.