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Gravitation is a chapter of a few formulas used again and again. Once you are sure which power of r belongs where, the questions become quick marks. Ten questions below, with the answer key and a hint for each.
What this chapter carries in NEET(UG)
- Usually 2 to 3 questions every year, which is 8 to 12 marks.
- Most of them come from the inverse square law, g at height and depth, orbital speed, escape speed and Kepler’s laws.
- The usual slip is mixing up r and r2 in the energy and force formulas.
What the syllabus says
In the syllabus (Unit 6, Gravitation): The universal law of gravitation, acceleration due to gravity and how it changes with altitude and depth, Kepler’s laws of planetary motion, gravitational potential energy and gravitational potential, escape velocity, motion of a satellite, orbital velocity, time period and energy of a satellite.
Taken out in 2024: geostationary satellites.
Taken from the NMC syllabus for NEET (UG) 2026, the latest one published. See the full syllabus, unit by unit.
Formulas you must know by heart
| Newton’s law of gravitation | F = Gm1m2/r2 |
| Acceleration due to gravity | g = GM/R2 |
| At a height h above the surface | g′ = g(1 − 2h/R), for h much smaller than R |
| At a depth d below the surface | g′ = g(1 − d/R) |
| Orbital speed | vo = √(GM/r) |
| Escape speed | ve = √(2GM/R) = √2 × vo |
| Escape speed from the earth | about 11.2 km/s |
| Kepler’s third law | T2 ∝ r3 |
| Gravitational potential energy | U = −Gm1m2/r |
Note the difference in the last two rows. Force goes as 1 over r squared, but potential energy goes as 1 over r.
Ten questions. Give yourself ten minutes.
Write your answers down before you scroll. A question you looked up is a question you have not answered.
Q1. If the distance between two bodies is doubled, the gravitational force between them becomes
- (a) half
- (b) one quarter
- (c) double
- (d) four times
Q2. The acceleration due to gravity at the centre of the earth is
- (a) zero
- (b) g
- (c) g/2
- (d) infinite
Q3. The escape speed from the surface of the earth is about
- (a) 7.9 km/s
- (b) 9.8 km/s
- (c) 11.2 km/s
- (d) 22.4 km/s
Q4. Escape speed is related to orbital speed near the surface by
- (a) ve = vo
- (b) ve = √2 vo
- (c) ve = 2 vo
- (d) ve = vo/2
Q5. Kepler’s third law states that the square of the period of a planet is proportional to
- (a) r
- (b) r2
- (c) r3
- (d) r4
Q6. The gravitational potential energy of two masses separated by r varies as
- (a) 1/r
- (b) 1/r2
- (c) r
- (d) r2
Q7. The value of g at a height equal to the radius of the earth above its surface is
- (a) g
- (b) g/2
- (c) g/4
- (d) zero
Q8. The weight of a body at the poles compared with its weight at the equator is
- (a) less
- (b) more
- (c) the same
- (d) zero
Q9. A satellite in a circular orbit is in a state of
- (a) no gravity acting on it
- (b) free fall, so apparent weightlessness
- (c) zero velocity
- (d) constant acceleration of zero
Q10. The escape speed of a body from a planet does not depend on
- (a) the mass of the planet
- (b) the radius of the planet
- (c) the mass of the body
- (d) the value of G
Answer key, with a hint for each
| Q1 | (b) | F goes as 1/r2. Double the distance and the force falls to one quarter. |
| Q2 | (a) | At the centre the pull from all directions cancels, so g is zero there. |
| Q3 | (c) | About 11.2 km/s from the earth’s surface. |
| Q4 | (b) | ve = √2 vo, which is why escape needs roughly 1.41 times the orbital speed. |
| Q5 | (c) | T2 ∝ r3. Cube of the radius, square of the period. |
| Q6 | (a) | U = −Gm1m2/r. The force goes as 1/r2 but the energy goes as 1/r. |
| Q7 | (c) | At height R the distance from the centre is 2R, so g becomes g/4. |
| Q8 | (b) | The earth is slightly flattened and there is no rotational effect at the poles, so weight is a little more there. |
| Q9 | (b) | Gravity is still acting. The satellite and everything in it are falling together, so they feel weightless. |
| Q10 | (c) | ve = √(2GM/R). The mass of the escaping body does not appear. |
Now mark yourself honestly
NEET(UG) marks the way it marks. Use the same rule here:
- +4 for a right answer
- −1 for a wrong one
- 0 for a blank
So ten questions here are out of 40. In the real paper, 45 Physics questions are out of 180.
If you scored 32 or more, the basics of this chapter are secure. Move to harder questions, not more of these.
If you scored below 24, do not feel bad about it. You have just found out something useful before the exam did. Go back to the formula table above, learn it properly, and sit these ten again tomorrow.
And if you guessed a few and they landed, count those as wrong. A guess that works once will not work in May.
The full chapter, on a clock
Ten questions tell you a little. Forty-five tell you the truth.
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