Is centripetal force a separate force in IB Physics?

No, centripetal force is not an additional physical interaction. It is the name given to the resultant force directed toward the centre of a circular path. Tension, gravity, friction, a normal force, or a combination of forces may provide this resultant. Draw the real forces first and then write their net inward component as $mv^2/r$. Adding a separate centripetal force arrow usually double-counts the force and leads to an incorrect equation.

Which radius should I use in a satellite question?

Use the distance from the satellite to the centre of the planet or other central body. If the question gives altitude $h$ above a planet of radius $R$, then the orbital radius is $r=R+h$. Using altitude alone underestimates the separation and affects gravitational force, speed, and period calculations. Check that both quantities use compatible units before adding them. A labelled sketch is often the quickest way to prevent this mistake.

Why can an object accelerate while moving at constant speed in a circle?

Acceleration describes a change in velocity, and velocity includes both magnitude and direction. In uniform circular motion, the speed remains constant but the velocity direction changes continuously. The resulting acceleration points toward the centre and has magnitude $v^2/r$. This acceleration requires a net inward force according to Newton’s second law. If that inward force disappears, the object moves tangentially rather than continuing around the circle.

Do I need to memorize the circular motion and gravitation equations?

The physics data booklet provides many relevant equations, but you must know what each symbol means and when each relationship applies. You should be able to derive orbital speed and period by combining gravitation with circular motion. Definitions such as gravitational field strength as force per unit mass must also be understood rather than merely located. Practising with the booklet is more useful than trying to memorize its layout immediately before an examination. Familiarity with derivations also helps when a question presents an unfamiliar arrangement.

How are circular motion and gravitation tested on IB Physics papers?

Paper 1A can test conceptual distinctions, proportional reasoning, force directions, and short calculations through multiple-choice questions. Paper 1B may place the topic in a data-based context involving graphs, measurements, or uncertainty. Paper 2 can require force diagrams, multi-step orbital calculations, derivations, explanations, and extended responses. Questions may also connect gravitation with energy, momentum, or experimental analysis. No individual topic is guaranteed to appear in a particular paper, so preparation should cover both conceptual and numerical forms.

What is the best way to revise circular motion and gravitation?

Begin by learning the small set of core ideas, especially radial acceleration, resultant inward force, gravitational field strength, and centre-to-centre distance. Then attempt questions without looking at notes or solutions. Watch each question worked through on video and compare the setup, not just the final numerical answer. Keep an error log that separates force-diagram, equation, algebra, unit, and interpretation mistakes. Finish by repeating similar questions under timed conditions until you can produce the method independently.

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IB Physics Circular Motion & Gravitation Questions

• 31 Jul 2026•11

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IB Physics Circular Motion & Gravitation Questions | RevisionDojo

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IB Physics Circular Motion & Gravitation Questions

• 31 Jul 2026•11

Circular motion and gravitation questions become much more manageable when you use one central method: identify the real forces, choose the inward radial direction, and apply Newton’s second law toward the centre. For an orbit, this usually means recognizing that gravity supplies the centripetal force, so $GMm/r^2=mv^2/r$ .

The same patterns recur across multiple-choice, data-based, short-answer, and extended-response questions. This guide explains those patterns, the equations behind them, and the mistakes that most often cost marks.

Where circular motion and gravitation appear in IB Physics

In the current IB Physics course, circular motion is principally studied within A.2 Forces and momentum, while gravitation appears in D.1 Gravitational fields. D.1 contains material for both SL and HL, with additional HL content involving gravitational potential, potential energy, equipotentials, escape speed, and orbital energy.

The current external assessment uses Paper 1A for multiple-choice questions, Paper 1B for data-based questions, and Paper 2 for short-answer and extended-response questions. According to the official IB Physics subject brief, Paper 1 contributes 36% and Paper 2 contributes 44% of the final grade, with the scientific investigation contributing the remaining 20%.

The IB does not guarantee that a particular topic will appear in a particular paper. However, the official specimen papers demonstrate the kinds of calculations, explanations, diagrams, and linked reasoning students must be ready to perform.

The essential equations and what they mean

Situation	Equation	Important interpretation
Angular and linear speed	$v=\omega r$	Use radians per second for $\omega$
Centripetal acceleration	$a_c=v^2/r=\omega^2r$

The physics data booklet gives many of these relationships, but it does not decide which equation fits a situation. Use the RevisionDojo Physics data booklet during practice so that finding and interpreting equations becomes automatic.

A reliable method for answering circular motion questions

Step 1: Identify the circular path

Mark the centre and determine the radius. The required acceleration and resultant radial force point toward the centre, even when the object’s instantaneous velocity is horizontal or tangential.

Step 2: Draw only real forces

Include forces such as weight, tension, normal contact force, friction, or gravity. Do not add a separate centripetal force arrow: centripetal force is the name for the resultant inward force, not an additional interaction.

Step 3: Choose inward as positive

Write a radial force equation before substituting values:

\sum F_{\text{inward}}=\frac{mv^2}{r}.

This prevents sign errors, particularly in vertical circles where the inward direction changes as the object moves.

Step 4: Resolve forces if necessary

Only radial components contribute to the centripetal acceleration. Tangential components change the object’s speed, whereas radial components change the direction of its velocity.

Step 5: Substitute units and calculate

Convert kilometres to metres, hours to seconds, and revolutions per minute to radians per second where needed. Keep extra calculator digits until the final answer.

Step 6: Check the physics

A force magnitude should not be negative. If solving for the minimum speed needed to maintain contact, set the relevant contact force or tension to zero at the limiting point.

Worked circular-motion example

A $0.50\,\text{kg}$ object moves at $6.0\,\text{m s}^{-1}$ in a horizontal circle of radius $2.0\,\text{m}$ . The resultant inward force is

F=\frac{mv^2}{r}=\frac{0.50(6.0)^2}{2.0}=9.0\,\text{N}.

This answer is the net radial force, not necessarily the tension. If tension is the only horizontal force, then the tension is 9.0 N; if several radial forces act, their resultant must equal 9.0 N.

For a vertical circle, direction matters. At the top, if both tension and weight point inward, $T+mg=mv^2/r$ . At the bottom, tension points inward but weight points outward, so $T-mg=mv^2/r$ . Memorizing one sign arrangement without drawing the forces is therefore unreliable.

Targeted practice in the circular motion questionbank helps make this setup routine. The accompanying circular motion topic resources can then be used to repair specific conceptual gaps.

How to solve orbital motion questions

For a satellite in a circular orbit, gravitational force is the real inward force:

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

Cancelling $m$ and one factor of $r$ gives

v=\sqrt{\frac{GM}{r}}.

This derivation explains two important results. The orbital speed does not depend on the satellite’s mass, and a satellite in a larger circular orbit travels more slowly.

Using $v=2\pi r/T$ produces

T^2=\frac{4\pi^2}{GM}r^3.

A frequent trap is to use altitude above the surface as $r$ . In gravitational and orbital equations, the required radius is normally

r=R_{\text{planet}}+h.

Students should also distinguish orbital speed from escape speed. Orbital speed maintains a particular circular orbit, while escape speed is the minimum initial speed required to reach an infinite distance with zero final speed when resistive forces are neglected.

Gravitational field questions

Gravitational field strength is defined as force per unit mass:

g=\frac{F}{m}=\frac{GM}{r^2}.

Because $g$ is a vector, fields from several masses must be added with directions considered. Along the line between two masses, their fields point in opposite directions, so a zero-field point may exist between them.

For masses $M_1$ and $M_2$ separated by distance $d$ , let the zero-field point be distance $x$ from . Set the field magnitudes equal:

\frac{GM_1}{x^2}=\frac{GM_2}{(d-x)^2}.

The factor $G$ cancels, giving

x=\frac{d\sqrt{M_1}}{\sqrt{M_1}+\sqrt{M_2}}.

The point lies closer to the smaller mass because a smaller distance is needed for its weaker field to match the stronger field. The D.1 gravitational fields questionbank provides focused practice with field strength, orbits, and related graph questions.

Additional HL relationships

HL students should be comfortable with gravitational potential energy and gravitational potential when zero is defined at infinity:

U=-\frac{GMm}{r}, \qquad V=-\frac{GM}{r}.

The negative sign indicates that a mass in a gravitational field is in a bound state relative to the zero at infinity. For a circular orbit,

K=\frac{GMm}{2r}, \qquad E_{\text{total}}=-\frac{GMm}{2r}.

Escape speed follows from setting the total energy at launch equal to zero:

v_e=\sqrt{\frac{2GM}{r}}.

Students often confuse potential, potential energy, and field strength. Potential is energy per unit mass, potential energy depends on the test mass, and field strength is related to the spatial rate of change of potential.

Common traps in IB Physics questions

Treating centripetal force as an extra force: write a resultant radial force equation instead.
Using altitude instead of orbital radius: measure $r$ from the planet’s centre.
Ignoring vector direction: gravitational fields and forces must be added vectorially.
Assuming constant speed means zero acceleration: direction changes continuously in circular motion.
Using (mgh) over planetary distances: (mgh) assumes an approximately uniform field.
Forgetting squared relationships: doubling $r$ reduces $g$ and gravitational force to one quarter.
Giving calculations without explanations: command terms such as “explain,” “derive,” and “show that” require connected reasoning.
Rounding too early: retain unrounded intermediate results to avoid avoidable numerical discrepancies.

The fastest way to improve your method

Re-reading notes can clarify definitions, but it rarely trains the sequence of decisions required under exam conditions. The fastest practical method is to attempt a question first, commit to a complete solution, and then watch the question worked through step by step.

When reviewing a worked video solution, record the first point where your method diverged. Classify the error as diagram, force identification, equation choice, algebra, units, or interpretation, then attempt a similar question without support. RevisionDojo’s per-question worked solutions and video explanations can be combined with the Physics Questionbank, while Physics predicted papers and video solutions provide longer timed practice.

Conclusion

Successful answers to IB Physics circular motion & gravitation questions begin with a force diagram and a clearly defined radial direction. In circular orbits, set gravitational force equal to the required centripetal force; in field problems, remember that gravitational field strength is a vector and that distance is measured from the centre of mass.

The recurring traps are predictable, so improvement comes from correcting methods rather than memorizing isolated answers. Use RevisionDojo’s Questionbank to attempt unfamiliar problems, then review per-question video solutions or ask Jojo AI to explain where your setup failed.

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Where circular motion and gravitation appear in IB Physics

The essential equations and what they mean

Situation	Equation	Important interpretation
Angular and linear speed	$v=\omega r$	Use radians per second for $\omega$
Centripetal acceleration	$a_c=v^2/r=\omega^2r$