Are the objects described here in static equilibrium, dynamic equilibrium, or not in equilibrium at all?
a. A girder is lifted at constant speed by a crane.
b. A girder is lowered by a crane. It is slowing down.
c. You're straining to hold a 200 lb barbell over your head.
d. A jet plane has reached its cruising speed and altitude.
e. A rock is falling into the Grand Canyon.
f. A box in the back of a truck doesn't slide as the truck stops.

a) In this case, the girder is in dynamic equilibrium, although there is movement it is realized at constant speed therefore there is no acceleration and the sum of forces is equal to zero.

b) There is no equilibrium since the velocity is decreasing it slows the girder, therefore the sum of forces will be equal to the product of mass by acceleration according to newton's second law.

c) In this case, the arms of the person are exercising each 100 lb, in order to keep the barbell stable, this allows the static equilibrium.

d) There is movement but however the jet will move at constant speed without any kind of acceleration, therefore it has a condition of dynamic equilibrium.

e) There is no balance as the rock falls and the acceleration due to gravity causes its speed to increase as it goes down.

f) In this case there is dynamic equilibrium as there was a deceleration movement however this force value given by the deceleration multiplied by the mass is not large enough to be able to move the box, due to the weight of the box plus the friction force between the box and the surface.