Universal Law of Gravitation - NEET Physics Class 11 Free MCQ Test

The correct answer is Option A - A, B and D only

• Weakest fundamental force: Gravitational interaction is far weaker than the other fundamental forces (strong, electromagnetic, weak); this is why it is described as the weakest fundamental force.

• Conservative force: The work done by gravity between two points is independent of the path taken, so the gravitational force is conservative. For any closed path, the net work by gravity is zero.

• Short-range - not true: gravity acts over arbitrary distances and is a long-range force, decreasing with distance but never becoming exactly zero.

  F = G m₁ m₂ / r²

  This inverse-square dependence shows that gravitational influence extends indefinitely, though it weakens with increasing r.

• Principle of superposition: Gravitational forces from multiple masses add vectorially; the net gravitational force is the vector sum of individual forces, so the principle of superposition holds.

• Can be shielded - not true: there is no known material or method that blocks or shields gravity; gravitational effects cannot be screened in the way electromagnetic fields can, so gravity is effectively unshieldable.

Option C is correct: the smaller body moves 7.5R before collision.

Let the smaller mass be m₁ = M and the larger mass be m₂ = 5M. The centres touch when their separation equals R + 2R = 3R, so the reduction in separation is 12R - 3R = 9R.

Let the displacement of the smaller mass towards the larger be Δx₁ and that of the larger mass towards the smaller be Δx₂. Because no external force acts, the centre of mass is fixed, giving the relation m₁Δx₁ = m₂Δx₂.

With m₁ = M and m₂ = 5M, this yields Δx₁ = 5Δx₂.

The total approach is Δx₁ + Δx₂ = 9R. Substituting gives 5Δx₂ + Δx₂ = 9R, so 6Δx₂ = 9R and Δx₂ = 1.5R.

Therefore Δx₁ = 5 × 1.5R = 7.5R. Hence the smaller body travels 7.5R before collision, corresponding to Option C.

The correct answer is Option B - Statement I is incorrect but Statement II is correct

The law of universal gravitation gives the force between two point masses as F = G m₁ m₂ / r², directed along the line joining them, where G is the gravitational constant.

Statement I is incorrect because the law itself is universal and applies to every pair of mass elements; the simple closed form F = G m₁ m₂ / r² is valid directly only for point masses or when an extended body is spherically symmetric and the force on an external point can be treated as if the entire mass were concentrated at the centre.

Statement II is correct because, for an extended body, the total gravitational force is obtained by the principle of superposition by summing (integrating) contributions of all mass elements. For a point mass m interacting with a continuous mass distribution with density ρ(r'), a differential force is dF = G m·dm / r² (with vector direction along the line joining the elements), and the net force is found by integrating over the body's volume.

In vector form, for a point at position r acted on by a continuous distribution ρ(r'), the force can be written as F = G m ∫ ρ(r') (r' - r) / |r' - r|³ dV', which implements the required integration over all mass elements.

Therefore, the correct assessment is that the first statement is false while the second statement is true, so Option B is correct.

The correct answer is Option D - 4 : 1

According to Newton's law of gravitation, the gravitational force between two point masses is given by F = G M m / r².

Let the force at the first distance be F₁ = G M m / r².

At twice the distance the force is F₂ = G M m / (2r)² = G M m / 4 r².

Therefore, the ratio F₁ : F₂ = (G M m / r²) : (G M m / 4 r²) which simplifies to 4 : 1.