Question

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.

Solution

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$\textbf{a)}$ As dynamic equilibrium represents equilibrium in the state of motion, then this case is indeed dynamic equilibrium. The man is at rest, but the crane lifts him so that he is in a state of motion.

$\textbf{b)}$ Not in equilibrium the crane slows down, does not move at a constant speed.

$\textbf{c)}$ If we hold the weight above our head so that we do not move, we do not have any change in speed then, this is a state of equilibrium and, it is static.

$\textbf{d)}$ The plane moves without changing direction. Therefore, without changing speed, its speed remains constant. Still, the plane is moving, so this is a dynamic equilibrium.

$\textbf{e)}$ The falling rock moves rapidly under the action of gravitational force. So we don't have a state of equilibrium, the speed is not constant.

$\textbf{f)}$ As the box does not move, then we have a static equilibrium.

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