1. 4.1 Work, Power and Energy pp. 86 - 90 Mr. Richter

2. Agenda (Today and Tomorrow)  Collect Lab Notebooks  Warm Up  Science Fair Proposal Rubric  Notes:  Work  Work and Energy  Power  Problem Solving Practice

3. Objectives: We Will Be Able To…  Define work in terms of force and distance.  Define work in terms of energy.  Calculate the work done when moving an object.  Explain the relationship between work and power.  Calculate power.

4. Warm-Up:  Student A walks up a flight of stairs. Student B runs up the same flight of stairs.  At the top of the stairs, which student, if either, has more potential energy?  Which student probably feels more tired? Why?  Discuss at your table and we will discuss as a class in a five minutes.

5. Work

6.  Work is a measure of how much energy is transferred.  Work is done (energy is transferred) by applying a force over a distance.  Note that work has the same units as energy: Joules. It takes energy to do work!

7. Work  Not all force does work on an object. Work is only done when force is applied in the same direction as an object’s motion.  Force A does no work because the block will not move.  Force B does a little work, because the box will move slightly.  Force C does the most work, because the box will move in the direction of the force.

8. Work and Energy

9. Warm - Up  A 40-kg girl sits on a sled at the top of a hill that is 12 meters high. Assuming no friction, how fast will she be going at the bottom of the hill? (This is a conservation of energy problem!)  Separate half-sheet of paper. 5 minutes.

10. Work and Energy  Doing work always means transferring energy.  If you do work to pull back a rubber band, you store energy in that rubber band that can released later.  If you do work to lift an object to a high shelf, you store gravitational potential energy.

11. Work and Energy  However, not all work increases the energy of an object.  Imagine sliding a box across the floor and then stopping.  Work must be done, because a force was applied for some distance.  But the energy of the box stays the same. It did not gain any height and because it stops, it has no kinetic energy.  The work you put in immediately turns energy into heat because of friction.

12. Work and Energy  To reiterate: the work done on an object is equal to the energy transferred.  This means:  the work done to lift an object is equal to the potential energy it gained during the lifting  the work done to speed up an object is equal to the kinetic energy it gained during the speeding up  How much work was done to lift a 20-kg barbell to a height of 2 m?

13. Work and Energy  The work done against gravity can also be thought of applying a force over a distance (W = Fd).  In this case the force is the weight of the object (force due to gravity) and the distance is the height the object reaches.  How much work is done to lift a 100-N barbell to a height of 1.5 meters?

14. Power

15.  Power is the rate at which work is done.  The more “powerful” something is, the faster it can transfer energy.  Remember the students on the stairs. They do the same amount of work, but Student B, who runs, uses his energy more quickly. His body needs time to recover.

16. Calculating Power  Power is the rate at which work is done, or how much work is done every second.  The unit of power is Watts [W]. A watt is equal to a Joule per second.  [1 W = 1 J/sec]

17. Calculating Power  A 50-kg student runs up a flight of stairs 8 meters high in 5 seconds. 1.How much potential energy did the student gain? 2.How much work did the student do? 3.How much power did the student have?

18. Wrap-Up: Did we meet our objectives?  Define work in terms of force and distance.  Define work in terms of energy.  Calculate the work done when moving an object.  Explain the relationship between work and power.  Calculate power.

19. Homework  p 90 #a, b, 1-4  Finish Conservation of Momentum Lab if you haven’t already