Work

1. Work Page 168

2. What is work? • A little different from what we’re use to in our everyday lives • When you hear the word work, what do you think of?

3. Scientific Definition of Work • A force that causes a displacement on an object does work on the object • Remember displacement … change in position (∆X = xfinal – xinitial) • Work = force x displacement W = Fd • Work is not done on an object unless the object is moved because of the action of the force

4. An example • Is work done in the following scenario? • 1. A student holds a heavy chair at arm’s length for several minutes. • No … why?

5. Parallel Components • Work is done only when components of a force are parallel to a displacement • When the force on an object and the object’s displacement are in different directions, only the component of the force that is in the direction of the object’s displacement does the work • Components of the force perpendicular to a displacement does not do work

6. For example … • Imagine pushing a crate along the ground • If the force you exert is horizontal, all of your effort moves the crate • If your force is other than horizontal, only the horizontal component of your applied force causes a displacement and does work • See Figure 5.2 page 169

7. Angles … • Look at Figure 5.2 page 169 • W = Fd (cos Θ) • If Θ = 0, the cos(0) = 1 • Therefore, W = Fd • If Θ = 90, cos (90) = 0, so work = 0 • Remember, work is only done if force is parallel

8. Another example Is work done in the following scenario? • 1. A student carries a bucket of water along a horizontal path while walking at constant velocity. • No … why?

9. What if many forces are acting on the object? • Then, • Wnet = Fnetd (cos Θ) • Net work = net force x displacement x cosine of angle between them

10. One last thing … • UNITS!! • Work has dimensions of force x length • In the SI system, newtons x meters = • joules (J) • Unit for work is the joule

11. Let’s check your assignment from yesterday (#1-4 page 171) • 1. For each of the following statements, identify whether the everyday or the scientific meaning of work is intended. • A. Jack had to work against time as the deadline neared. • B. Jill had to work on her homework before she went to bed. • C. Jack did work carrying the pail of water up the hill.

12. Answers • 1. • A. Everyday sense • B. Everyday sense • C. Scientific sense

13. #2 • If a neighbor pushes a lawnmower four times as far as you do but exerts only half the force, which one of you does more work and by how much?

14. Answer • 2. • The neighbor; twice as much

15. #3 • For each of the following cases, indicate whether the work done on the second object in each example will have a positive or negative value. • A. the road exerts a friction force on a speeding car skidding to a stop. • B. A rope exerts a force on a bucket as the bucket is raised up a well. • C. Air exerts a force on a parachute as the parachutist slowly falls to Earth.

16. Answer • 3. • A. Negative • B. Positive • C. Negative

17. #4 • Determine whether work is being done in each of the following examples: • A. a train engine pulling a loaded boxcar initially at rest. • B. a tug of war that is evenly matched. • C. a crane lifting a car

18. Answer • 4. • A. Yes • B. No • C. Yes