Question

When a body is in equilibrium, which of the following must be true?
(A) There is no force acting on it.
(B) The sum of all the forces acting on it must be zero.
(C) The sum of all the forces acting on it is not equal to zero.
(D) There are exactly two forces acting on the body.

Answer 1

Hint:The equilibrium is the condition of the state of the object that remains stable when no force acts on it or when two or more forces act but still it remains in the balanced condition. The equilibrium condition is mainly applicable to the objects only when it is in rest.

Complete step by step solution:
The body is said to be in equilibrium only when the force acting on it is zero. This zero force condition is possible only when zero force acting on it or else the forces acting on the object is balanced by them itself that may be two or more than two.

Let u consider that no force acts on the object,

$F = 0$ ------(1)

Let us consider that the two forces that are equal but act opposite in direction act on the same object. The magnitude of the force is considered as $5$ . The sum of the force is calculated as follows.

$F = 5 + \left( { - 5} \right) = 0$ ------(2)

The equation (1) and (2) represents the equilibrium condition since the total or the sum of the force acting on the object is zero. This is said by Newton's first law of motion. Hence the body is said to be in equilibrium if there is no force acting on it or else the sum of all the forces acting on the object is zero.

Thus the options (A) and (B) are correct.

Note:Newton’s first law of motion gives the idea about the equilibrium of the object. It says the body continues its balanced condition until any external unbalanced forces are applied on it. This equilibrium is maintained until the unbalanced force acts on it.