Is centripetal force a separate force in IB Physics?

No. Centripetal force describes the net force directed toward the centre of a circular path, rather than an additional physical interaction. Depending on the situation, it may be provided by gravity, tension, friction, a normal force, or a combination of forces. Your free-body diagram should contain only genuine interactions acting on the selected object. After drawing them, add their radial components and set the result equal to $mv^2/r$.

Should I use altitude or distance from the planet's centre in gravitation equations?

Use the distance measured from the centre of the planet or spherical mass. If an object is at altitude $h$ above a planet of radius $R$, then the required distance is $r=R+h$. The same centre-to-centre distance is used in $F=GMm/r^2$, $g=GM/r^2$ and circular-orbit equations. Using altitude alone overestimates gravitational force and orbital speed, sometimes by a large amount. Marking the planet's centre and radius on a diagram is an effective way to prevent this error.

Why do astronauts feel weightless when gravity still acts on them?

Astronauts and their spacecraft are both falling toward Earth under gravity. They have enough tangential velocity for Earth's curved surface to fall away beneath them, so they remain in orbit. Because the astronauts and spacecraft share nearly the same acceleration, the spacecraft does not provide the usual supporting normal force. A scale would therefore read approximately zero, producing apparent weightlessness. Saying that there is no gravity in orbit is incorrect because gravity provides the centripetal acceleration.

Does a heavier satellite orbit faster than a lighter satellite?

For circular orbits at the same radius around the same central mass, both satellites have the same orbital speed. Equating gravitational and centripetal forces gives $GMm/r^2=mv^2/r$. The satellite mass $m$ cancels, leaving $v=\sqrt{GM/r}$. This result assumes that the satellite's mass is negligible compared with the central body and that other forces can be ignored. A more massive satellite has a larger gravitational force, but it also has proportionally greater inertia.

What is the best way to review circular motion and gravitation video solutions?

Attempt each question independently before opening the solution, even if you are unsure how to finish it. During the video, pause after the diagram and predict the force equation before the presenter reveals it. Compare the chosen radius, radial direction, force components and unit conversions with your own work. Record one precise reason for every lost mark instead of writing a general note such as “revise gravity.” Finally, repeat the question without assistance after a delay to confirm that you can reproduce the method rather than merely recognize it.

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IB Physics Circular Motion & Gravitation Mistakes

Q: Why is an object accelerating if its speed is constant?

Acceleration is the rate of change of velocity, and velocity includes both magnitude and direction. During uniform circular motion, the magnitude of velocity remains constant while its direction changes continuously. The resulting centripetal acceleration points toward the centre and has magnitude $v^2/r$. It is perpendicular to the instantaneous velocity, so it changes the direction of motion without changing the speed. This distinction between speed and velocity is essential in both calculations and written explanations.

• 31 Jul 2026•11

IB Physics Circular Motion & Gravitation Mistakes

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IB Physics Circular Motion & Gravitation Mistakes | RevisionDojo

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IB Physics Circular Motion & Gravitation Mistakes

• 31 Jul 2026•11

The most frequent IB Physics circular motion & gravitation common mistakes come from treating centripetal force as a separate force, using the wrong radius, confusing speed with velocity, mishandling inverse-square relationships, and forgetting that orbiting objects are in free fall. These errors are usually conceptual rather than mathematical, so simply memorizing more equations rarely fixes them.

A better method is to review worked solutions one step at a time: identify the physical system, draw the forces, choose a radial direction, write Newton's second law, and only then substitute values. The current IB Physics course places circular motion within A.2 Forces and momentum and gravitational fields within D.1 Gravitational fields, with orbital motion connecting the two areas.

Equations you must interpret correctly

The central equations are straightforward, but each applies only when its variables and directions are understood.

Relationship	Meaning	Frequent misuse
$a_c=\frac{v^2}{r}=\omega^2r$	Radial acceleration toward the centre	Treating it as tangential acceleration
$\sum F_r=\frac{mv^2}{r}$
$v=\omega r=\frac{2\pi r}{T}$	Links linear speed, angular speed and period	Using diameter instead of radius
$F_g=\frac{GMm}{r^2}$
$g=\frac{GM}{r^2}$	Gravitational field strength outside a spherical mass	Assuming $g$ is always
$v_{orbit}=\sqrt{\frac{GM}{r}}$
$T^2=\frac{4\pi^2}{GM}r^3$

The official IB Physics specimen papers for first examinations in 2025 illustrate the current assessment structure. Students should practise applying these relationships in unfamiliar contexts rather than expecting questions that ask only for direct substitution.

Mistake 1: Treating centripetal force as an additional force

Students often draw gravity, tension and a separate arrow labelled “centripetal force.” This double-counts the force responsible for the inward acceleration. Centripetal force is not a new interaction; it is the name given to the net radial force.

For a satellite in a circular orbit, gravity provides the centripetal force:

$\frac{GMm}{r^2}=\frac{mv^2}{r}$

For a car on a flat bend, static friction may provide it. For a mass on a string, tension may provide all or part of it.

Fix: In worked video solutions, pause immediately after the force diagram. Check that every arrow represents a genuine interaction, then write the inward components of those forces as $\sum F_r=mv^2/r$ .

Mistake 2: Assuming circular motion means constant velocity

In uniform circular motion, speed is constant but velocity is not. Velocity is a vector, so its continuously changing direction means the object is accelerating even when a speedometer reading remains unchanged.

Centripetal acceleration points toward the centre and is perpendicular to the instantaneous velocity. It changes direction rather than speed, so the associated net radial force does no work in ideal uniform circular motion.

Fix: Draw a tangential velocity arrow and a radial acceleration arrow at the object's current position. The RevisionDojo circular motion notes can be used to review this geometry before attempting calculations.

Mistake 3: Using the wrong radius

In $v^2/r$ , $GMm/r^2$ and orbital equations, $r$ is measured from the centre of the circular path or spherical mass. If a satellite is at altitude above a planet of radius , the correct orbital radius is

$r=R+h.$

Using $h$ alone can produce a drastically incorrect result. A similar error occurs when students substitute a circle's diameter for its radius.

Fix: Before choosing an equation, mark the relevant centre and draw the radial distance. In a multi-step worked solution, verify how the radius is defined before copying any numerical value.

Mistake 4: Ignoring the direction and combination of forces

The expression $mv^2/r$ is the net inward force, not necessarily the magnitude of one force. At the bottom of a vertical circle, for example, the inward direction is upward, giving

$T-mg=\frac{mv^2}{r}.$

At the top, both tension and weight may point inward, producing $T+mg=mv^2/r$ . Reusing the same equation at every position is therefore incorrect.

Fix: Redraw the force diagram whenever the object's position changes. Choose inward as positive locally, resolve each genuine force along that direction, and keep signs consistent.

Mistake 5: Mishandling inverse-square relationships

Gravitational force and field strength vary as $1/r^2$ , not $1/r$ . If the distance from a planet's centre doubles, gravitational field strength becomes one quarter of its original value:

$\frac{g_2}{g_1}=\left(\frac{r_1}{r_2}\right)^2.$

Students also sometimes assume that moving one planetary radius above the surface halves $g$ . In fact, the centre-to-centre distance has doubled, so $g$ becomes one quarter of the surface value, assuming a spherical planet.

Fix: Use a ratio before inserting constants. The D.1 gravitational fields resources and gravitational field notes provide targeted review of field strength and distance relationships.

Mistake 6: Thinking astronauts in orbit experience no gravity

Astronauts appear weightless because they and their spacecraft are accelerating together in continuous free fall, not because gravity has disappeared. At typical orbital distances, Earth's gravitational field remains substantial.

The spacecraft's sideways velocity prevents it from striking the surface as it falls. Gravity continually bends its trajectory, supplying the inward acceleration required for orbit.

Fix: When explaining apparent weightlessness, use the ideas of common acceleration and zero normal contact force. Avoid saying that there is “no gravity” unless the field is genuinely negligible.

Mistake 7: Missing cancellations in orbital derivations

Equating gravity with the required centripetal force gives

$\frac{GMm}{r^2}=\frac{mv^2}{r}.$

The satellite mass $m$ cancels, leading to $v=\sqrt{GM/r}$ . Students who retain $m$ may incorrectly conclude that heavier satellites need a different circular-orbit speed at the same radius.

Fix: Keep equations symbolic until the physical dependencies are clear. Practise the derivation through the orbital motion and Kepler's laws questionbank, then explain verbally why satellite mass cancels.

Mistake 8: Substituting too early and losing units

Premature substitution hides cancellations, increases calculator errors and makes it harder for an examiner to follow the reasoning. Common unit mistakes include using kilometres instead of metres, hours instead of seconds, or a mass in grams.

Fix: Write the governing equation, rearrange symbolically, convert every value to SI units, and substitute only at the end. Keep unrounded calculator values during intermediate steps and attach an appropriate unit to the final answer.

How to learn from worked video solutions

Watching a solution passively is less effective than predicting each step. Use this routine with the A.2.4 circular motion questionbank and per-question solutions:

Attempt the question under timed conditions.
Identify the centre of motion and define the radial direction.
Draw only genuine forces acting on the chosen object.
Pause the video before the equation is shown and predict it.
Compare your setup, not just your numerical answer, with the worked method.
Record the exact cause of the error, such as “used altitude instead of orbital radius.”
Retry the question without notes several days later.

Use the broader IB Physics Questionbank to mix circular motion and gravitation with other mechanics questions. Jojo AI can help identify whether a lost mark came from the physical model, algebra, units or explanation.

A reliable exam method

For most calculation questions, apply the sequence system, forces, direction, law, algebra, units. Start by identifying the object under analysis and what provides its radial acceleration. Draw the force diagram, choose inward as positive, and apply Newton's second law without inventing an extra centripetal force.

After calculating, test whether the answer is physically sensible. A larger orbital radius should produce a lower circular-orbit speed but a longer period, while a stronger net inward force should produce greater centripetal acceleration for the same mass.

Conclusion

Circular motion and gravitation errors cluster around force identification, vector direction, radius definitions, inverse-square reasoning and orbital interpretation. They are best corrected by studying complete worked methods, pausing before each step, and then reproducing the reasoning independently.

RevisionDojo's topic questionbanks, notes and per-question video solutions can support this process. Use the circular motion and gravitational fields resources to diagnose one recurring mistake at a time rather than repeatedly completing questions without reviewing why marks were lost.

Sources and referenced URLs

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