Satellites do fall toward Earth. An orbit is continuous free fall in which a satellite has enough tangential velocity to keep missing the planet as Earth’s surface curves away beneath it. Gravity does not hold the satellite up or disappear in space. Instead, gravity continuously changes the direction of the satellite’s velocity, producing the centripetal acceleration required for orbital motion.
This explanation connects several ideas that IB Physics questions commonly test: Newton’s law of gravitation, circular motion, gravitational field strength, orbital speed, period, energy, and atmospheric drag. The emphasis here is the single central concept behind orbiting satellites; for broader coverage, use the topic-wide IB Physics Circular Motion and Gravitation Explained guide.
The short answer: satellites are constantly falling
Imagine throwing a ball horizontally from a high mountain. Gravity pulls it downward while its horizontal velocity carries it forward. If it is thrown faster, it travels farther before reaching the ground.
Now imagine an ideal Earth with no atmosphere and a mountain high enough to avoid all obstacles. At a sufficiently large horizontal speed, the ball falls toward Earth at the same time as Earth’s curved surface falls away beneath it. It never reaches the surface, so it continues around the planet.
This is sometimes called Newton’s cannonball thought experiment. It shows that orbiting and falling are not different types of motion. An orbit is a particular kind of fall in which the object continually misses the body toward which it is falling.
A satellite therefore needs two things:
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Gravity, which accelerates it toward Earth’s centre.
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Tangential velocity, which carries it sideways rather than directly toward the surface.
Tangential velocity is not an additional force. It is the satellite’s instantaneous velocity, directed along the tangent to its orbital path. Without gravity, the satellite would follow that tangent in a straight line; with gravity, its path continually curves toward Earth.
Gravity provides the centripetal force
For a satellite of mass m in a circular orbit around Earth, the gravitational force is
F = GMm/r²
where:
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G is the universal gravitational constant
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M is Earth’s mass
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m is the satellite’s mass
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r is the distance from Earth’s centre to the satellite
The satellite is moving in a circle, so it requires a resultant force directed toward the centre. The required centripetal force is
F = mv²/r
Gravity is the real force providing this inward resultant. Equating the expressions gives
GMm/r² = mv²/r
After cancelling m and one factor of r:
v = √(GM/r)
This is the speed required for a circular orbit of radius r around a much more massive central body of mass M.
A crucial exam point is that centripetal force is not a separate force acting alongside gravity. “Centripetal” describes the direction and role of the resultant force. In the ideal satellite model, the gravitational force itself is the centripetal force.
The satellite’s mass cancels from the equation. At the same orbital radius, satellites of different masses require the same circular orbital speed, provided that their masses are negligible compared with Earth’s mass.
For a fuller treatment of the circular-motion framework, review the RevisionDojo notes on circular motion.
Why the satellite accelerates even at constant speed
In a circular orbit, the satellite’s speed can remain constant while its velocity changes. Velocity is a vector, so changing its direction counts as acceleration even when its magnitude is unchanged.
The velocity vector is always tangent to the circular path. The gravitational force and centripetal acceleration point radially inward, toward Earth’s centre. These vectors are perpendicular at every instant.
Because the force is perpendicular to the motion, gravity changes the direction of the velocity rather than its magnitude in an ideal circular orbit. This is why the satellite can accelerate continuously while maintaining constant speed.
QuantityDirection in a circular orbitWhat it doesVelocity, vTangent to the orbitCarries the satellite sidewaysGravitational force, FToward Earth’s centreCurves the trajectoryCentripetal acceleration, aToward Earth’s centreChanges the direction of velocityDisplacement over a short intervalApproximately tangentialAdvances the satellite around the orbit
Students sometimes describe orbit as a “balance between gravity and velocity.” This can be a useful informal picture, but it must not be interpreted as two opposing forces. Velocity is not a force, and gravity is not balanced by an outward force in an inertial Earth-centred frame. There is a non-zero inward resultant force, which is why the satellite accelerates.
Gravity is still strong in orbit
Satellites do not remain in orbit because they have travelled beyond Earth’s gravity. A gravitational field extends indefinitely, although its strength decreases with distance according to
g = GM/r²
The International Space Station and other low-Earth-orbit spacecraft are only a few hundred kilometres above Earth’s surface. Since Earth’s mean radius is about 6,371 km, their distance from Earth’s centre is only moderately larger than the surface radius. Gravity there is therefore still a substantial fraction of its surface value.
Astronauts appear weightless because the spacecraft, astronauts, and objects inside are all accelerating together under gravity. This condition is called free fall or microgravity. It is not the absence of gravity.
An astronaut standing on Earth is supported by a normal contact force from the ground. In an orbiting spacecraft, there is no comparable support force acting continuously, so the astronaut and spacecraft fall together and no ordinary support force is felt.
Orbital radius means distance from Earth’s centre
One of the most frequent IB Physics errors is using altitude h in an equation that requires orbital radius r. Gravitational and circular-motion equations use the centre-to-centre distance:
r = Rₑ + h
where Rₑ is Earth’s radius and h is the satellite’s altitude above the surface.
For example, a satellite 400 km above Earth has an orbital radius of approximately
r = 6,371 km + 400 km = 6,771 km
or 6.771 × 10⁶ m in SI units. Substituting 400 km directly for r would make the gravitational force and orbital speed far too large.
Worked example: orbital speed in low Earth orbit
Consider a satellite in a circular orbit 400 km above Earth. Use:
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Earth’s mass, M = 5.97 × 10²⁴ kg
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Earth’s mean radius, Rₑ = 6.37 × 10⁶ m
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G = 6.67 × 10⁻¹¹ N m² kg⁻²
First calculate the orbital radius:
r = 6.37 × 10⁶ + 4.00 × 10⁵ = 6.77 × 10⁶ m
Then apply the circular orbital-speed equation:
v = √(GM/r)
v = √
v ≈ 7.67 × 10³ m s⁻¹
The satellite must therefore travel at about 7.7 km s⁻¹. This agrees with the European Space Agency’s explanation that spacecraft in a roughly 300 km circular orbit require a speed close to 7.8 km s⁻¹.
The orbital period follows from speed equals distance divided by time:
T = 2πr/v
Using the calculated values gives a period of approximately 5.55 × 10³ s, or about 92 minutes. The satellite falls around the entire planet in a little over an hour and a half.
Higher orbits are slower, not faster
The equation
v = √(GM/r)
shows that increasing orbital radius decreases the circular orbital speed. This sometimes seems counterintuitive because reaching a higher orbit requires a rocket to transfer energy to the satellite. However, after the satellite has settled into the higher circular orbit, it moves more slowly than it did in the lower circular orbit.
A higher orbit also has a longer circumference. Combining the speed equation with T = 2πr/v gives
T = 2π√(r³/GM)
Therefore:
T² ∝ r³
This is the circular-orbit form of Kepler’s third law. A satellite farther from Earth travels around a larger path at a lower speed, so its period increases significantly.
Circular-orbit changeOrbital speedOrbital periodGravitational field strengthRadius increasesDecreasesIncreasesDecreasesRadius decreasesIncreasesDecreasesIncreases
The RevisionDojo orbital motion and Kepler’s laws notes develop these relationships without replacing the central free-fall explanation given here.
Circular and elliptical orbits
The derivation v = √(GM/r) assumes a circular orbit, so the orbital radius and speed remain constant. Real satellites can also follow elliptical orbits, with Earth at one focus of the ellipse.
In an elliptical orbit, the satellite’s distance from Earth and its speed both change. It moves fastest at perigee, its closest point to Earth, and slowest at apogee, its farthest point. Gravity has both perpendicular and parallel components relative to the satellite’s instantaneous velocity for much of the orbit, so it can change both the direction and magnitude of the velocity.
Elliptical satellites are still falling continuously. The phrase “falling and missing Earth” applies to any unpowered bound orbit, not only a perfect circle. The simple circular equation should not, however, be applied at an arbitrary point in an elliptical orbit.
Students who need more practice distinguishing circular and elliptical cases can use the D.1.2 orbital motion videos and the associated orbital motion questionbank.
What happens if the speed is wrong?
At a specified radius, one particular tangential speed produces a circular orbit. Other velocities can produce different trajectories.
Initial conditionLikely result in the ideal modelNo tangential velocitySatellite falls nearly radially toward EarthToo slow for a circular orbitPath curves inward, often becoming an ellipse that may intersect EarthCorrect circular speedCircular orbit at constant radiusFaster than circular speed but below escape conditionsElliptical orbit with the release point near perigeeEscape speed or greater, directed appropriatelyUnbound trajectory away from Earth
For the same radius, escape speed is
vₑ = √(2GM/r)
which is √2 times the circular orbital speed. “Faster than circular speed” therefore does not automatically mean escape. A range of speeds can produce bound elliptical orbits.
Why real satellites can eventually fall
The ideal orbital model assumes that gravity is the only significant force. Real low-orbit satellites encounter a very thin upper atmosphere, which produces drag. Drag removes mechanical energy and angular momentum from the orbit.
As the orbit decays, its altitude decreases and the satellite encounters denser atmosphere. This increases drag, accelerating the decay until the object re-enters. Spacecraft may use thrusters periodically to raise or correct their orbits, but they do not normally need continuous thrust merely to keep moving forward.
Other effects can perturb an orbit, including:
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Earth’s non-spherical mass distribution
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gravitational forces from the Moon and Sun
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solar radiation pressure
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interactions with the upper atmosphere
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deliberate manoeuvres by onboard propulsion
These effects explain why “satellites never fall” is not strictly correct. Satellites are always falling gravitationally, while some also lose altitude over time because non-conservative forces alter their orbital energy.
For HL students, gravitational potential and orbital motion provide the energy-based explanation, while the broader IB Physics Fields HL exam guide connects orbital energy with potential, escape speed, and field strength.
IB Physics exam expectations
In the current IB Physics course, first assessed in 2025, D.1 Gravitational fields includes gravitational fields, Newton’s law of gravitation, Kepler’s laws, and orbital motion. The official physics data booklet lists F = Gm₁m₂/r² and g = GM/r² for the shared SL and HL material, while gravitational potential, escape speed, and the stated orbital-speed equation appear in the additional HL material.
Even when an equation is available, exam questions can ask you to explain or derive the result. A reliable method is:
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Draw Earth, the orbit, and the satellite.
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Mark velocity tangent to the path.
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Draw gravitational force toward Earth’s centre.
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Convert altitude into centre-to-centre radius.
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State that gravity provides the centripetal force.
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Write GMm/r² = mv²/r and simplify.
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Check units and whether the trend is physically reasonable.
Avoid these common statements:
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“There is no gravity in space.” Gravity is the cause of the orbit.
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“Centrifugal force balances gravity.” In the usual inertial frame, there is a net inward force.
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“Velocity balances gravity.” Velocity is not a force.
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“The satellite has no acceleration.” Its velocity direction changes continuously.
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“Centripetal force and gravity both act inward.” Gravity is providing the centripetal resultant.
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“Use the height as r.” The equations require distance from Earth’s centre.
A strong written explanation might say: The satellite is in continuous free fall. Its tangential velocity carries it forward while Earth’s gravitational force produces centripetal acceleration toward Earth’s centre, so its trajectory curves around Earth and continually misses the surface.
Conclusion
Satellites do not avoid falling; orbit is continuous free fall. Their tangential velocity carries them sideways while gravity supplies the inward centripetal force that bends their path around Earth. For a circular orbit, equating gravitational and centripetal force gives v = √(GM/r), showing that higher circular orbits have lower speeds but longer periods.
For exam preparation, focus on vector directions, centre-to-centre radius, and the distinction between a real force and the centripetal role of that force. After reviewing the concept, practise with RevisionDojo’s Newton’s law of gravitation questionbank, using Jojo AI to identify gaps in explanations and equation setup.
