Find the work contributed by each force

[vu10758]

This problem comes with a diagram so I have it at this link

http://viewmorepics.myspace.com/index.cfm?fuseaction=viewImage&friendID=21023924&imageID=1274314408

It is problem number 3. I think that the work contributed by tension is zero since the force of tension is perpendicular to the motion of the pendulum in this case. Air resistance is negative since it is acting against the motion. What is the work contributed by the force of gravity. The answer key says that it is positive for part a and negative for part b, but why?

 

[arildno]

It is easiest to understand this by remembering "power" as the time derivative of work, and rewrite Newton's 2.law to reflect that.
(this is done by multiplyting N2law with the velocity)
Then we get the equation:
$$\frac{dW}{dt}=\frac{dK.E}{dt}, K.E=\frac{m}{2}\vec{v}^{2}$$
That is, when you start out with zero work done, and increase it (getting positive), then you INCREASE the kinetic energy of your object!

Relate this to the answers for gravity on b) and c)

 

[BishopUser]

Work = |Force| * |Distance| * Cos(angle between them)

in A-B the angle between the distance and force vectors goes from 0-90, and cos is always positive between 0-90. From B-C the angle goes from 90-180, and the cos is always negative from 90-180.