On the problems due last week, many people (incorrectly) used energy to solve the second problem. While that seems like a good idea, the work term that people used was flawed. Since the ball is rolling while slipping, it is more complicated than just Fd where d is the distance along the lane.

Our rotation test is on Wednesday, think of this as good practice for the final. This unit covers alot of what we've seen before in linear systems- kinematics, dynamics, energy and momentum.

Our last week or so will be spent applying all that we have learned this semester to the motion of the planets and other astronomical bodies.

Today, we'll study equilibrium or static situations. These are ones where the system is at rest. We've dealt with similar systems when we only considered linear motion. There we saw that "at rest" measnt that the sum of the external forces had to be zero. Now we add to that condition, the net external torques must also sum to zero.

The primary challenge here will be to draw accurate free-body diagrams. We must take care to "attach" the forces at the places where they are actually applied. This will affect the torque that they produce. (Also, like all rotational systems, we must specify the pivot point.)