CK-12-Physics - Intermediate

http://www.ck12.org Chapter 6. Work and Energy

θ= 180 ◦; cos 180◦=−1 and the force is said to be doing negative work upon the object. 3. The force is perpendicular

to the object, making it turn but not speed up or slow down, so

θ= 90 ◦; cos 90◦=0 and no work is done upon the object.

In general, for any angle [0, 90) between the force and displacement, positive work is done upon the object. For any

angle (90, 180] between the force and displacement, negative work is done upon the object.

FIGURE 6.2

Positive work is done when is in the range

[0, 90) with the respect to the displace-

ment.

FIGURE 6.3

Negative work is done when is in the

range (90, 180] with respect to the dis-

placement.

Computing Work

Illustrative Example 6.1

InFigure6.5 a man is pushing a crate with a force of 500 N over a distance of 3.0 meters.

How much work does the man do on the crate?

W= (Fcosθ)x, where the angleθbetweenFandxis 0◦.

Answer:W= (500N)(3m) =1500 N·m.

One thing we immediately see from this exercise is the units of work.