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. Answer: 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. Answer: 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. Answer: 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. Answer: STATIC EQUILIBRIUM
Statement a and e are in static equilibrium, b and d are in dynamic equilibrium and c is not at equilibrium at all.
When the net force acting on an object is zero, the object is said to be at equilibrium. The state of equilibrium can be classified into two: static and dynamic.
Static equilibrium is when the net force is zero resulting in the object being at rest or not moving.
Dynamic equilibrium is when the resultant force acting on an object is zero and the object is moving in a uniform motion (i.e. constant or unchanging speed).
A. a 200 pound barbell is held above your head
<em>Since the barbel is "held above your head" and is implied to be at rest since there is no change in its position during the time it is held, then this situation is an example of static equilibrium.</em>
B. A girder is being lifted at a constant speed by a crane
<em>This is dynamic equilibrium because the object is moving "at a constant speed".</em>
C. A girder is being lowered into place. It is slowing down.
<em>This is not equilibrium condition because the objects is neither at rest nor moving at a constant speed. It is "slowing down", so the speed is decreasing.</em>
D. A jet plane has reached its cruising speed and altitude.
<em>The "cruising speed" is the speed that is maintained while the jet plane is traveling because this is considered to be the most efficient speed. Since it is maintained, the jet plane is said to be in constant motion. Therefore, this is dynamic equilibrium.</em>
E. A box in the back of a truck doesn't slide as the truck stops.
<em>This is static equilibrium because the box is stationary or not moving.</em>
For many solids<span> dissolved in liquid water, the </span>solubility increases<span> with </span>temperature<span>. The </span>increase<span> in kinetic energy that comes with </span>higher temperatures<span> allows the solvent molecules to more effectively break apart the solute molecules that are held together by intermolecular attractions.</span>