A box of mass 500 gm is lying on a horizontal table. Which of the following…

Concept

Newton's First Law states that a body continues in its state of rest (or uniform motion) unless an unbalanced (net) external force acts on it. When two or more forces act on a body such that their resultant is zero, they are called a pair of balanced forces, and the body's state of rest or motion does not change.

Application

1. Convert the mass to SI units: m = 500 g = 0.5 kg.

2. Find the weight (the force due to gravity) acting downward on the box: W = mg = 0.5 × 10 = 5 N.

3. The box is described as lying at rest on the table — that is, remaining stationary over time with no other applied forces — so by Newton's first law (a body free of net external force stays at rest) the net force acting on it must be zero.

4. For the net force to be zero, the table must exert a normal (reaction) force of 5 N upward on the box, exactly matching its weight.

5. These two forces — 5 N weight acting downward and 5 N normal force acting upward — act on the same box, are equal in magnitude and opposite in direction, and add up to zero net force: this is exactly a pair of balanced forces.

Cross-check

If the box actually experienced a net unbalanced force of 5 N in either direction, it would accelerate away from its resting position (by F = ma, at 5 / 0.5 = 10 m/s²) and would not remain lying still on the table over time — contradicting the given description. Gravity also always acts on any object with mass, so the claim that no force at all acts on the box cannot be right either. Only a pair of balanced forces is consistent with the box remaining at rest on the table.