A state of equilibrium is when the body is:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Net force acting on the body is zero.
A body is in equilibrium when all the forces acting on it balance, so that the net (resultant) force on it is zero. By Newton’s first law, it then keeps its state: it stays at rest, or it keeps moving in a straight line at a constant speed.
Why the other options are incorrect
Option (1) At rest — Incorrect. Rest is only one case of equilibrium; a body moving at a constant velocity is in equilibrium too.
Option (2) In motion — Incorrect. A moving body is in equilibrium only if its velocity does not change; one that speeds up, slows down or turns is not.
Option (3) Falling freely — Incorrect. A freely falling body has an unbalanced force, its weight, acting on it and keeps speeding up.