Gravitation MCQs for NEET — Physics Questions with Answers

According to the NCERT text, 'The force of attraction between a hollow spherical shell of uniform density and a point mass situated outside is just as if the entire mass of the shell is concentrated at the centre of the shell.' This is a key result for spherical mass distributions.

The NCERT states, 'The force of attraction due to a hollow spherical shell of uniform density, on a point mass situated inside it is zero.' This is a fundamental property of gravitational fields within uniform spherical shells.

The NCERT explicitly mentions, 'For a spherically symmetric body however the force on a particle external to the body is as if the mass is concentrated at the centre and this force is therefore central.'

The NCERT states, 'If a particle is inside a homogeneous solid sphere, the force on the particle acts toward the centre of the sphere. This force is exerted by the spherical mass interior to the particle.' Gravitational forces from shells outside the particle's radius cancel out.

The NCERT explains, 'For a point inside the earth, ...The point P lies outside the sphere of radius r. For the shells of radius greater than r, the point P lies inside. Hence according to result stated in the last section, they exert no gravitational force on mass m kept at P. The shells with radius $\le r$ make up a sphere of radius r for which the point P lies on the surface.' Therefore, only the mass within the radius $r$ contributes.

Based on the NCERT text, 'On the other hand the mass of the sphere $M_r$ of radius $r$ is $\frac{4}{3}\pi r^3 \rho$'. This mass acts as if concentrated at the center.

From the NCERT explanation for a point inside the Earth (homogeneous solid sphere), the force is given by $F = G \frac{m M_r}{r^2}$. Since $M_r = \frac{4}{3} \pi r^3 \rho$, substituting this gives $F = G \frac{m (\frac{4}{3} \pi r^3 \rho)}{r^2} = G m (\frac{4}{3} \pi \rho) r$. Thus, the force is proportional to $r$.

The NCERT states, 'The force of attraction due to a hollow spherical shell of uniform density, on a point mass situated inside it is zero. Qualitatively, we can again understand this result. Various regions of the spherical shell attract the point mass inside it in various directions. These forces cancel each other completely.'

The NCERT states, 'at a point outside the earth, the gravitational force is just as if its entire mass of the earth is concentrated at its centre.' This applies for any spherically symmetric body.

The NCERT clearly states under 'EXERCISES', '(a)You can shield a charge from electrical forces by putting it inside a hollow conductor. Can you shield a body from the gravitational influence of nearby matter by putting it inside a hollow sphere or by some other means ?' and later, 'Gravitational shielding is not possible.' This highlights the difference between gravitational and electrical forces in this regard.