Work and Energy

1. Work and Energy pp 55–61

2. What’s the point? Energy is critically important to Nature. (It’s more fundamental than mass, perhaps as fundamental as space and time.)

3. Objectives • Relate work to force and distance. • Calculate an object’s gravitational potential energy near the earth’s surface. • Calculate the kinetic energy of a moving object.

4. Check Question Work is • Force  time. • Force  distance. • Force / distance. • Force / time. • Mass  velocity.

5. Work Formula work = F·Dd F= force appliedDd= displacement (distance traveled)

6. kg m kg m2 J = Nm = m = s2 s2 Units of Work joule (J) = 1 newton along 1 meter

7. Group Work Estimate the work done by the strong man in the video. Show your work! Justify your estimates of force and distance.

8. Can You See Why The work required to lift a mass m a height h above the earth’s surface is W = mgh Hint: What is the force applied? What is the distance traveled?

9. Potential Energy The energy of relative position of two objects gravity springs electric charges chemical bonds = the work you can get from the force.

10. Gravitational Potential Energy Gravitational potential energy = the work to raise an object to a height Gravitational PE = mgh = the work gravity does lowering the object

11. component of force in direction of motion Work is a Scalar Source: Griffith, The Physics of Everyday Phenomena

12. A. 100 N 100 N D. 0 m 10 m B. 100 N 100 N 100 m 100 m E. 10 N C. 100 m Group Work • Rank the following scenarios from least work done to most work done.

13. Group Question The piglet has a choice of three slides to descend. Along which slide would gravity do the most work on the piglet? A B C D. Same work for all. E. Need more information.

14. Group Question The piglet has a choice of three slides to descend. Along which slide would friction do the most work on the piglet? A B C D. Same work for all. E. Need more information.

15. Question To accelerate an object from 10 to 20 m/s requires • more work than to accelerate from 0 to 10 m/s. • the same amount of work as to accelerate from 0 to 10 m/s. • less work than to accelerate from 0 to 10 m/s.

16. It Takes Work Source: Griffith, The Physics of Everyday Phenomena to speed something up.

17. v = F area = Dd slope = a = speed v m t t time • Work = F·Dd F = m (slope) = mv mv 1 1 Dd = vt t t 1 2 2 mv2 • Work = vt = 2 Work of Acceleration • To accelerate to speed v with constant force F

18. mv2 1 2 K = Kinetic Energy the work to bring a motionless object to speed

19. 10 m/s 5 kg 10 m/s 5 kg 40 m/s 10 kg 10 kg 20 m/s Question Which has the greatest kinetic energy? A. B. C. D.

20. initial v initial v final v final v= 0 Rebound and Stop

21. Question Which changes its momentum the most? • A moving object that stops when it hits a barrier. • A moving object that bounces back from a barrier. Hint: Momentum has direction.

22. Question Which changes its kinetic energy the most? • A moving object that stops when it hits a barrier. • A moving object that bounces back from a barrier. Hint: kinetic energy depends on speed, not direction.

23. Work-Energy Theorem • If net work W is done on a system, the system’s kinetic energy changes by DK = W.

24. W = Dt DE Power = Dt Power Rate of doing work DE=change in energy ( = work) Dt=time interval

25. DE Power = Dt Energy kg m2 kg m2 W = = time s2 s s3 Units of Power = J/s = W = watt

26. Group Work The 2004 Tour de France’s Alpe d’Huez time trial stage was a steep climb with its finish 1200 m higher than the start. Lance Armstrong won with a timeDt of 39:41 (2381 s). He and his gear had a combined mass of 84 kg. What was Lance’s average powerDE/Dt during the stage? Hint: Use change in gravitational potential energy for DE.

27. Reading for Next Time pp 62–65 Forms of energy Converting between forms