Are the objects described here in static equilibrium, dynamic equilibrium, 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.