Question 7: Why a particle experiencing only one force cannot be in equilibrium?
In case a body to be in equilibrium, we know one of the two conditions is that the algebraic sum of all forces acting on the body is zero. Mathematically, ∑ F = 0.
Now this is possible when forces of equal magnitudes acting along the same line of action but in opposite directions are acting on the body. Since there is a single force acting on the body and no other force to balance it, therefore, it will produce acceleration in the body (in accordance to Newton’s second law of motion) and the body will not be in equilibrium.