a. A car driving in a straight line at 20 m/s.
Explanation:
An object is in a state of equilibrium when no force is acting upon it. There are two types of equilibrium; static equilibrium and dynamic equilibrium.
Static equilibrium is a state when a body is at rest.
Dynamic equilibrium is an equilibrium state when a body is moving at a constant velocity. (Rectilinear Motion).
A car moving in a straight line at 20 m/s has a constant velocity and hence no force is acting on it. So, it is in dynamic equilibrium.
A book sitting on a table without moving is not is dynamic equilibrium but in static equilibrium.
A boy jumping off a diving board in not in equilibrium as gravitational force is acting upon him and he has a changing velocity.
A motorcycle going in a circle at a constant speed has changing velocity because the direction of the motion is constantly changing hence it is not in the state of motion.
Keywords: velocity, force, equilibrium, static equilibrium, dynamic equilibrium
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