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Equilibrium and Motion

Equilibrium

Observe the animated image. The top ball is stationary while the bottom ball is moving at a constant velocity. But both balls are in a state of equilibrium. But how can the moving ball be in a state of equalibrium? An object is said to be in equilibrium if its state of motion remains unchanged. This condition exists only if all forces acting on the object are balanced. If the forces are unbalanced, the object will accelerate or decelerate. This is not the case for either ball.

Examples

A ball sitting on the crest of a hill is in a state of equilibrium. But if a strong wind causes the ball to move, equilibrium is broken by the unbalanced force.
A rope being pulled in a tug-of-war is at equilibrium as long as both sides are pulling with equal force; it will not move. Even though force is being applied through the "pulling" action, the equilibrium is stable.
A car traveling down a road at 60 mph is at equalibrium. The push pf the engine is balanced by the resistance of the road and the air. If the car is given more gas, the force of the engine increases causing an unbalanced force. The car will then accelerate.