A body is observed to be at rest at a certain instant. Which conclusion is most justified?
AIt must be in equilibrium because its velocity is zero
BIt cannot have any force acting on it
CIt may or may not be in equilibrium; one must check the net force and net moment
DIt must have zero acceleration
Answer & Solution
Correct answer: C. It may or may not be in equilibrium; one must check the net force and net moment
The underlying principle is that equilibrium is determined by forces and moments, not by an instantaneous value of velocity.
1. A body can have zero velocity at one instant and still accelerate afterward.
2. For equilibrium, both $\vec F_{\text{net}}=0$ and net moment $=0$ must hold.
3. The example of a ball at the highest point of its upward motion shows this clearly: its velocity is momentarily zero, but gravity still acts, so it is not in equilibrium.
Why the distractors are tempting: A is a common misconception that rest implies equilibrium, but momentary rest is not enough. B is false because forces can act even when velocity is zero. D is false because zero velocity at an instant does not guarantee zero acceleration.
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