GravitationNEET Physics · Class 11 · NCERT Chapter 7

You can expect 1 to 3 questions from Gravitation in NEET 2027. The chapter has high PYQ frequency. Kepler's third law, escape velocity, orbital velocity, the variation of g with altitude or depth, and total energy of a satellite are tested almost every year.

Every two point masses attract each other with a force F equals G m1 m2 over r squared, directed along the line joining them. G is the universal gravitational constant, equal to 6.67 times 10 to the minus 11 N m squared per kg squared. The law works for spherical bodies treating them as if all mass were at the centre.

At height h above the surface, g_h equals g times R squared over (R plus h) squared, smaller than the surface value. At depth d below the surface, g_d equals g times (1 minus d over R), also smaller. At the centre of Earth, g equals zero. NEET asks both of these almost every year.

Escape velocity is the minimum speed an object needs to leave the gravitational pull of a planet without further propulsion. It equals square root of (2 G M over R), or equivalently square root of (2 g R). For Earth, v_esc is approximately 11.2 km per second. It does not depend on the mass of the escaping object.

For a satellite in a circular orbit at height h, the orbital velocity is v_orbital equals square root of (G M over (R plus h)). For low Earth orbit (h is much smaller than R), this simplifies to about 7.9 km per second. The orbital velocity equals escape velocity divided by square root of 2, regardless of orbit radius near Earth.

A geostationary satellite has its orbital period equal to one sidereal day (24 hours), so it appears stationary above one point on Earth. This requires it to orbit in the equatorial plane at a height of about 36,000 km, where v_orbital equals about 3.07 km per second. Used for communications and TV broadcasting.

Kepler's third law says T squared is proportional to r cubed for any planet or satellite around a central body. The constant of proportionality is 4 pi squared over G M, where M is the mass of the central body. NEET problems often give T and r for one orbit and ask for the other, which only requires this proportionality.