Are the objects described here in equilibrium while at rest, in equilibrium while in motion, or not in equilibrium at all? Explain.

a. A 200 pound barbell is held over your head.
b. A girder is lifted at constant speed by a crane.
c. A girder is being lowered into place. It is slowing down.
d. A jet plane has reached its cruising speed and altitude.
e. A box in the back of a truck doesn’t slide as the truck stops.