Are the objects described here in static equilibrium, dynamic equilibrium, or not equilibrium at all? explain. a. a 200 pound barbell is held above your head b. a girder is being lifted at a constant speed by a crane c. a girder is being lowered into place. its is slowing down d. a jet plane has reached its cruising speed an altitude e. a box in the back of a truck doesn't slide as the truck stops

Let us examine the given situations one at a time.

Case a. A 200-pound barbell is held over your head.
The barbell is in static equilibrium because it is not moving.
STATIC EQUILIBRIUM

Case b. A girder is being lifted at a constant speed by a crane.
The girder is moving, but not accelerating. It is in dynamic equilibrium.
DYNAMIC EQUILIBRIUM

Case c: A jet plane has reached its cruising speed at an altitude.
The plane is moving at cruising speed, but not accelerating. It is in dynamic equilibrium.
DYNAMIC EQUILIBRIUM

Case d: A box in the back of a truck doesn't slide as the truck stops.
The box does not slide because the frictional force between the box and the floor of the truck balances out the inertial force. The box is in static equilibrium.
STATIC EQUILIBRIUM