Satellite Energy

8 MCQs4 revision cards0-step worked example

Source: NCERT GravitationPYQ coverage: NEET 2021, 2022, 2023, 2024, 2025Official key: NTA-verifiedLast reviewed: May 2026

Lesson

The energy of an orbiting satellite is a bound-state accounting problem — and the trap that costs marks is forgetting one side of the ledger.

The core relationship. For a satellite of mass m in a circular orbit at radius r = R + h around Earth (mass M):

Gravitational PE: U = −GMm/r
Orbital KE: K = GMm/(2r)
Total mechanical energy: E = K + U = −GMm/(2r)

The total energy is exactly −K, or equivalently U/2. This is the virial theorem result for inverse-square forces. A bound satellite always has E < 0. If energy is added until E = 0, the satellite escapes — it reaches the threshold of an unbound orbit.

The high-frequency trap. When a question asks "how much energy is needed to move a satellite from orbit 1 to orbit 2," aspirants commonly compute only the PE change (ΔU) and forget that the satellite must also change its orbital speed. The correct answer is the change in total energy: ΔE = E₂ − E₁. Since E includes both KE and PE, you cannot ignore either. Similarly, the energy to launch a satellite from rest on the surface to a circular orbit at height h is not just the PE gain — you must also supply the orbital KE.

Sign discipline. E is negative and its magnitude decreases as r increases (the satellite becomes less tightly bound at higher orbits). "More energy" means E becomes less negative — closer to zero, not more negative.

NCERT Class 11 Physics, Chapter 8 (Gravitation), page 14 derives the satellite energy formula by combining the orbital velocity condition with the PE expression.

Practice MCQs

Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.

MCQ 1Easy RecallPractice

For a satellite in a stable circular orbit around Earth, which statement about its total mechanical energy E is correct?

MCQ 2Easy RecallPractice

For a satellite in circular orbit, what is the relationship between its kinetic energy K and total mechanical energy E?

MCQ 3Easy RecallPractice

For a satellite of mass *m* in a circular orbit of radius *r* around a planet of mass *M*, the gravitational potential energy is U = −GMm/r. What is the ratio |U|/K, where K is the orbital kinetic energy?

MCQ 4Direct ApplicationPractice

A satellite orbits Earth in a circular orbit of radius *r*. If it is moved to a new circular orbit of radius 2*r*, what is the ratio of its new total energy E₂ to the original total energy E₁?

MCQ 5Direct ApplicationPractice

A satellite of mass *m* is in a circular orbit at height *h* = *R* above Earth's surface (orbit radius = 2*R*). What is its total mechanical energy? (Take *g* as surface gravitational acceleration, *R* as Earth's radius.)

MCQ 6Direct ApplicationPractice

A satellite is in a circular orbit with total energy E. What additional energy must be supplied to make it escape to infinity?

MCQ 7CalculationPractice

A satellite of mass *m* rests on Earth's surface. What minimum energy must be supplied to place it in a circular orbit at height *h* = *R* above the surface? (Neglect air resistance. Use *g* for surface gravity, *R* for Earth's radius.)

MCQ 8CalculationPractice

Two satellites A and B of masses *m* and 2*m* orbit Earth in circular orbits of radii 2*R* and 4*R* respectively. What is the ratio of total energy of A to that of B, i.e., E_A/E_B?

Quick recall before you leave

Before solving, remember these

Formula

Energy of a satellite

Total mechanical energy E = -G M m / (2(R+h)) = -KE = U/2 (virial relations for circular orbit). Negative E means the satellite is bound; making E ≥ 0 escapes.

-- NCERT Class 11 Physics, Ch. 7, p. 14

Formulas

8 formulas — click to collapse

Escape velocity from a body's surface

v_{e} = \frac{2 GM}{R} = 2 g R

Minimum speed for an object to escape gravity to infinity from radius R. Earth: ~11.2 km/s.

Symbol	Quantity	SI Unit
v_e	escape velocity	m/s
M	planet mass	kg
R	planet radius	m
g	surface gravity	m/s^2

Valid when

Launched from surface
No air drag
Body treated as point/sphere

Gravitational potential energy (point masses)

U = - \frac{GM m}{r}

PE of two-body system; negative because gravity is attractive (work to separate them is positive).

Symbol	Quantity	SI Unit
U	grav PE	J
M, m	two masses	kg
r	separation	m

Valid when

Reference U=0 at r=infinity
Point or spherical masses

g variation with altitude

g_{h} = g (\frac{R}{R + h})^{2} \approx g (1 - 2 h / R)

Gravitational acceleration decreases with altitude above Earth's surface.

Symbol	Quantity	SI Unit
g_h	g at height h	m/s^2
g	surface g	m/s^2
R	Earth radius	m
h	altitude	m

Valid when

Static observer at altitude
Earth treated as uniform sphere

g variation with depth

g_{d} = g (1 - d / R)

Inside Earth (uniform density), g decreases linearly with depth, vanishing at centre.

Symbol	Quantity	SI Unit
g_d	g at depth	m/s^2
g	surface g	m/s^2
R	Earth radius	m
d	depth	m

Valid when

Earth treated as uniform density sphere

Kepler's third law

T^{2} = \frac{4 π ^{2}}{GM} a^{3}

Square of orbital period proportional to cube of semi-major axis. Holds for elliptic orbits about a central mass.

Symbol	Quantity	SI Unit
T	orbital period	s
a	semi-major axis	m
M	central mass	kg

Valid when

Two-body system with central mass M >> orbiting mass
Bound orbit

Orbital velocity for circular orbit

v = \frac{GM}{R + h}

Speed of circular orbit at altitude h above body of mass M, radius R.

Symbol	Quantity	SI Unit
v	orbital speed	m/s
M	central mass	kg
R+h	orbit radius	m

Valid when

Circular orbit
M >> orbiting mass

Satellite total mechanical energy

E = - \frac{GM m}{2 ( R + h )}

Total energy = KE + PE = -KE (virial). Always negative for bound orbit; E -> 0 at infinity.

Symbol	Quantity	SI Unit
E	total energy	J
M, m	central mass and satellite mass	kg
R+h	orbit radius	m

Valid when

Circular orbit
Bound (E < 0)

Newton's law of gravitation

F = \frac{G m _{1} m _{2}}{r ^{2}}

Attractive force between any two masses. Inverse-square central force.

Symbol	Quantity	SI Unit
F	force	N
G	grav constant = 6.674e-11	N*m^2/kg^2
m1, m2	masses	kg
r	centre-to-centre distance	m

Valid when

Point masses or spherically symmetric distributions
r > sum of body radii (else use shell theorem)

Exam Traps & Common Mistakes

These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.

4 items — click to collapse

Trap

g at altitude: linear vs inverse-square

Category: Similar Terms

Student treats g(h) as linear in h. Actual: g(R/(R+h))² (inverse-square).

When it triggers

Question asks for g at significant altitude (e.g. R/2 above surface).

How to avoid

Use g_h = g (R/(R+h))². Linear approximation g(1 - 2h/R) only valid for h << R.

Trap

Kepler's third law: linear vs 3/2 power

Category: Similar Terms

Student treats T proportional to a (linear) instead of a^(3/2).

When it triggers

Question gives change in semi-major axis and asks for new period.

How to avoid

T² ∝ a³, so T ∝ a^(3/2). Doubling a multiplies T by 2^(3/2) ≈ 2.83.

Mistake

Treats g(h) as linear in h for significant heights.

Root cause: formula misuse

Correction

Use g_h = g(R/(R+h))² (inverse-square). Linear approximation g(1-2h/R) is only valid for h << R. For h = R/2, the exact formula gives g_h = (2/3)² g = 4g/9, not g(1-1) = 0.

Mistake

Treats T proportional to a (linear) instead of a^(3/2) in Kepler's third law.

Root cause: formula misuse

Correction

T² ∝ a³, so T ∝ a^(3/2). Doubling a multiplies T by 2^(3/2) ≈ 2.83. Quadrupling a → 8× T.

Past Year Questions

8 questions from NEET 2021, 2022, 2023, 2024, 2025. Answers verified against NTA official keys. — click to collapse

NEET 2025

The radius of Martian orbit around the Sun is about 4 times the radius of the orbit of Mercury. The Martian year is 687 Earth days. Then which of the following is the length of 1 year on Mercury?

1124 earth days

288 earth days

3225 earth days

4172 earth days

NTA Answer: Option 2(final)

NEET 2025

A body weighs 48 N on the surface of the earth. The gravitational force experienced by the body due to the earth at a height equal to one-third the radius of the earth from its surface is :

136 N

216 N

327 N

432 N

NTA Answer: Option 3(final)

NEET 2024Revised key

1 The mass of a planet is th that of the earth and its diameter is half that of the earth. The acceleration due 10 to gravity on that planet is:

119.6 m s–2

29.8 m s–2

34.9 m s–2

43.92 m s–2

NTA Answer: Option 4(revised_final)

NEET 2024Revised key

The minimum energy required to launch a satellite of mass m from the surface of earth of mass M and radius R in a circular orbit at an altitude of 2R from the surface of the earth is: 5GmM

16R 2GmM

23R GmM

32R GmM

43R

NTA Answer: Option 1(revised_final)

NEET 2023

Two bodies of mass m and 9m are placed at a distance R. The gravitational potential on the line joining the bodies where the gravitational field equals zero, will be (G = gravitational constant) 12Gm 16Gm

1−

2− R R 20Gm 8Gm

3−

4− R R

NTA Answer: Option 2(final)

NEET 2023

A satellite is orbiting just above the surface of the earth with period T. If d is the density of the earth and G is 3π the universal constant of gravitation, the quantity represents Gd

1T2

2T3

NTA Answer: Option 1(final)

NEET 2022

A body of mass 60 g experiences a gravitational force of 3.0 N, when placed at a particular point. The magnitude of the gravitational field intensity at that point is

1180 N/kg

20.05 N/kg

350 N/kg

420 N/kg

NTA Answer: Option 3(final)

NEET 2021

The escape velocity from the Earth's surface is v. The escape velocity from the surface of another planet having a radius, four times that of Earth and same mass density is

14v

32v

43v

NTA Answer: Option 1(final)

How NEET usually asks this

4 recurring patterns from past papers — click to collapse

Compare escape velocities for planets with different M, R. v_e ∝ sqrt(M/R). Common shape: 'planet has 4× radius and 2× density of Earth — find v_e'.

Direct ApplicationEasy

Common distractors

forgets density not mass

Confuses planet mass with density given radius scaling

Body weighs W on surface; find weight at height h above surface. Use g_h = g(R/(R+h))^2.

Direct ApplicationEasy

Common distractors

uses linear decrease with h

Treats g as linear in h instead of inverse-square

Given period T and semi-major axis a of one planet, find for another with different a. T^2 ∝ a^3.

Direct ApplicationEasy

Common distractors

uses linear relation

Treats T proportional to a not a^(3/2)

Energy required to launch satellite to altitude. Compute change in total mechanical energy.

Multi StepMedium

Common distractors

forgets orbital KE component

Computes only PE change, ignoring orbital KE

Test yourself on this topic with real past-paper questions:

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Orbital Velocity Satellite Time Period

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Satellite Energy

Lesson

Practice MCQs

Quick recall before you leave

Satellite total mechanical energy

Gravitational PE (point masses)

Orbital velocity

Surface gravity substitution

Before solving, remember these

Formulas

Valid when

Valid when

Valid when

Valid when

Valid when

Valid when

Valid when

Valid when

Exam Traps & Common Mistakes

When it triggers

How to avoid

When it triggers

How to avoid

Correction

Correction

Past Year Questions

How NEET usually asks this

Common distractors

Common distractors

Common distractors

Common distractors

Free NEET study resources