Equilibrium Requires Zero Net Force

Steps:

• Recall Newton's First Law: An object remains at rest or in uniform motion unless acted on by a net force.
• Define equilibrium: A state where velocity is constant (zero or non-zero), implying no change in motion.
• Identify the condition for constant velocity: The vector sum of all forces (resultant force) must be zero.
• Eliminate options that contradict this by not ensuring zero net force.

Why D is correct:

• By Newton's First Law, equilibrium occurs when the resultant force is zero, preventing acceleration and maintaining constant velocity.

Why the others are wrong:

• A: Stationary objects are in equilibrium, but moving objects with constant velocity also qualify, so it's not the single must condition.
• B: Constant non-zero acceleration means unbalanced forces, violating equilibrium.
• C: No forces allows equilibrium but ignores cases with balanced forces canceling out.

Final answer: D