1. Chapter 6 Work & Energy

2. Work Two thing are involved with work: –The application of force –The movement caused by force W = Fd Units (Nm) or J (joules)

3. Work Push a box across the floor. When a force is exerted over a distance, work is done. Work is the product of force and distance when the two are in the same direction. dF

4. Work and Direction Consider a sled being dragged across the snow by a diagonal rope. The angle between the quantities is denoted as  What is the direction of motion of the sled? What is the direction of the force acting on the sled? d F

5. Work and Direction (cont.) Only the horizontal component of the force is acting in the direction of motion d F d F Fy Fx

6. Sample Work Problem 2 The box (m = 15kg) is now lifted a height of 1.5m. How much work is done? The force required to lift an object is equal to its weight. F Lift d

7. Practice Problem 2 (Work) A Lawnmower is being pushed across the grass. The angle of the handle is 50° with the horizontal. The job requires the a force of 80N across the 33m yard. How much work is done? F d

8. When No Work Is Done Carry a box horizontally across the room. What is the direction of motion? What is the direction of the force? What is the angle  ? What is d F So when force and displacement vectors are perpendicular, no work is done.

9. Work And Friction Friction requires work to be done. Consider a box being dragged across a sidewalk. What is the direction of the displacement? What is the direction of the frictional force? What is the angle between the vectors below? What is cos 180°? This means that work can be negative. This may include situations other than friction. d F

10. Work Energy Theorem WET=ΔKE How much work is required to accelerate a 1100kg car from 0m/s to 50m/s?

11. Power Work is independent of time. –Lift a 10N box 2m in 2s W=20J –Lift a 10N box 2m in 4s W=20J The equation for power is shown below. Power (W) Time (s) Work (J)

12. Machines Terms Input Work (W i ): –The work you do Output Work (W o ): –The work the machine does.

13. Mechanical Advantage Mechanical advantage is the ratio of output force to input force. This shows how many times a force is multiplied by a machine. –If MA = 1, then no forces are multiplied. –If MA > 1, then forces are multiplied. –If MA < 1, then forces are divided. The trade off is distance If you cut the force in half, the distance doubles FOFO FIFI dIdI dOdO

14. Pulleys The approximate MA for a pulley system is the number of supporting strands.

15. Pulleys The approximate MA for a pulley system is the number of supporting strands.

16. Pulleys The approximate MA for a pulley system is the number of supporting strands.

17. Pulleys The approximate MA for a pulley system is the number of supporting strands.

18. Efficiency