What does Gravitational Fields cover in IB Physics?

This topic describes gravitational fields, including Kepler's laws of orbital motion and Newton's law of universal gravitation. Kepler's three laws describe the motion of planets around the Sun. Newton's universal law of gravitation states that F = G(m_1m_2)/(r^2).

Is Gravitational Fields SL or HL?

Both. SL and HL students study Gravitational Fields, and HL goes further: that the gravitational potential energy E_p of a system is the work done to assemble the system from infinite separation of its components.

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FourtyFive has 19 Gravitational Fields questions. Every answer you write is marked mark by mark, IB-style, and you see where each mark was won or lost. Every part has a hint, the AI tutor helps you through the step you are stuck on, and your Study Profile picks what to practise next.

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Topic D.1 · SL and HL

Gravitational Fields: notes and practice questions

Q: How do I revise Gravitational Fields for IB Physics?

Start from the core idea: this topic describes gravitational fields, including Kepler's laws of orbital motion and Newton's law of universal gravitation. In the exam: both papers, both levels. May 2025 Paper 2 TZ1 question 2 was common to SL and HL: calculate an orbital radius ratio using Kepler's third law from two orbital periods (2 marks), then explain how planetary observations let scientists determine the mass of the Sun (2 marks, "any 2 from" three marking points). Then practise exam-style questions, easiest first, writing out every step of your working before you check it.

8 min read Jump to 19 practice questions

Summary

This topic describes gravitational fields, including Kepler's laws of orbital motion and Newton's law of universal gravitation.
Kepler's three laws describe the motion of planets around the Sun.
Newton's universal law of gravitation states that $F = G\frac{m_1m_2}{r^2}$ .
Gravitational field strength $g$ is defined as the force per unit mass, $g = \frac{F}{m} = G\frac{M}{r^2}$ .
Extended bodies can be treated as point masses for gravitational calculations under specific conditions.
Gravitational field lines are used to represent the direction and strength of a gravitational field.
Orbital motion problems at this level assume circular orbits.

How it is examined

Both papers, both levels. May 2025 Paper 2 TZ1 question 2 was common to SL and HL: calculate an orbital radius ratio using Kepler's third law from two orbital periods (2 marks), then explain how planetary observations let scientists determine the mass of the Sun (2 marks, "any 2 from" three marking points). HL got a third part using the potential gradient: estimate the average field strength from $\Delta V/\Delta r$ (2 marks). That part is HL-only content and shows how the same stem is split across levels. Potential, potential energy, escape speed and equipotentials must never appear in an SL question.

Given in the booklet

SL: $F = Gm_1m_2/r^2$ , $g = F/m = GM/r^2$ , and the gravitational constant G. Kepler's three laws are statements, not booklet equations, and the third law in the form $r^3/T^2 = \text{constant}$ has to be assembled by the student. HL adds $E_\text{p}$ , $V_\text{g}$ , the potential gradient, $W = m\Delta V_\text{g}$ , $v_\text{esc}$ and $v_\text{orbital}$ .

Key ideas

Kepler's three laws of orbital motion
Newton's universal law of gravitation as given by $F = G\dfrac{m_1 m_2}{r^2}$ for bodies treated as point masses
conditions under which extended bodies can be treated as point masses
that gravitational field strength g at a point is the force per unit mass experienced by a small point mass at that point, as given by $g = \dfrac{F}{m} = G\dfrac{M}{r^2}$

At HL

that the gravitational potential energy $E_\text{p}$ of a system is the work done to assemble the system from infinite separation of its components
the gravitational potential energy for a two-body system as given by $E_\text{p} = -G\dfrac{m_1 m_2}{r}$ , where r is the separation between the centres of mass of the two bodies
that the gravitational potential $V_\text{g}$ at a point is the work done per unit mass in bringing a mass from infinity to that point, as given by $V_\text{g} = -G\dfrac{M}{r}$
the gravitational field strength g as the gravitational potential gradient, as given by $g = -\dfrac{\Delta V_\text{g}}{\Delta r}$

Guiding questions

How are the properties of a gravitational field quantified?
How does an understanding of gravitational fields allow for humans to explore the solar system?

Linking questions

What measurements of a binary star system need to be made in order to determine the nature of the two stars?
How is uniform circular motion like and unlike real-life orbits?
How is the amount of fuel required to launch rockets into space determined by considering energy?
How can air resistance be used to alter the motion of a satellite orbiting Earth?
What are the benefits of using consistent terminology to describe different types of fields? (NOS)
How can the motion of electrons in the atom be modelled on planetary motion and in what ways does this model fail? (NOS)
Physics utilizes a number of constants such as G. What is the purpose of these constants and how are they determined? (NOS)

Practice questions

19 questions · 5 easy · 14 medium

Showing 19 of 19

Question 1

EasyPaper 1A · calculator1 mark

Two satellites, A and B, are in stable circular orbits around the Earth. The orbital period of satellite A is $T_A$ and its orbital radius is $R_A$ . The orbital period of satellite B is $T_B$ and its orbital radius is $R_B$ . Which expression correctly relates their periods and radii?

A. $\frac{T_A}{T_B} = \frac{R_A}{R_B}$

B. $\frac{T_A^2}{T_B^2} = \frac{R_B^3}{R_A^3}$

C. $\frac{T_A^2}{T_B^2} = \frac{R_A^3}{R_B^3}$

D. $\frac{T_A^3}{T_B^3} = \frac{R_A^2}{R_B^2}$

Question 2

MediumPaper 1A · calculator1 mark

In a simplified classical model of an atom, an electron orbits the nucleus in a circular path. Electron A orbits the nucleus at a radius $R$ . Electron B orbits the same nucleus at a radius $2R$ . Assume the only force acting on the electrons is the electrostatic force from the nucleus, which provides the centripetal force for the orbit. This electrostatic force follows an inverse square law with distance.

What is the ratio $\frac{\text{centripetal acceleration of electron A}}{\text{centripetal acceleration of electron B}}$ ?

A. $\frac{1}{4}$

B. $\frac{1}{2}$

C. 2

D. 4

Question 3

EasyPaper 1A · calculator1 mark

Two artificial satellites, A and B, are in stable circular orbits around the Earth.

The orbital period of satellite A is $T_A$ and its orbital radius is $r_A$ . The orbital period of satellite B is $T_B$ and its orbital radius is $r_B$ .

What is the ratio $\frac{r_A}{r_B}$ ?

A. $\left(\frac{T_A}{T_B}\right)^{\frac{2}{3}}$

B. $\left(\frac{T_A}{T_B}\right)^{\frac{3}{2}}$

C. $\left(\frac{T_B}{T_A}\right)^{\frac{2}{3}}$

D. $\left(\frac{T_B}{T_A}\right)^{\frac{3}{2}}$

Question 4

MediumPaper 1A · calculator1 mark

An exoplanet, Kepler-X, has a mass that is 8 times the mass of another exoplanet, Kepler-Y. The radius of Kepler-X is twice the radius of Kepler-Y.

The gravitational field strength at the surface of Kepler-Y is $g$ .

What is the gravitational field strength at the surface of Kepler-X?

A. $0.5g$

B. $2g$

C. $4g$

D. $32g$

Question 5

EasyPaper 1A · calculator1 mark

What is the primary application of the stellar parallax method in astronomy?

A. To determine the chemical composition of a star.

B. To measure the distance to nearby stars.

C. To calculate the surface temperature of a star.

D. To measure the recessional velocity of distant galaxies.

Question 6

MediumPaper 2 · calculator4 marks

(a)

A space probe, carrying a scientific instrument, lands on an exoplanet. The instrument has a weight of $750\ \text{N}$ on the surface of Earth.

(a) Calculate the weight of the instrument on the exoplanet if the exoplanet has a radius that is one-third of Earth's radius and a density that is $1.5$ times the density of Earth.

[2]

(b)

(b) Calculate the weight of the instrument on a different celestial body, Planet Y, if Planet Y has a radius that is $0.8$ times Earth's radius and a mass that is $0.6$ times Earth's mass.

[2]

Question 7

EasyPaper 1A · calculator1 mark

A communications satellite is in a stable elliptical orbit around the Earth. It is moving from its perigee (the point of closest approach) to its apogee (the point of furthest approach).

Which quantity is increasing during this part of the orbit?

A. The satellite's speed

B. The satellite's kinetic energy

C. The satellite's gravitational potential energy

D. The magnitude of the gravitational force on the satellite

Question 8

MediumPaper 2 · calculator5 marks

(a)

A geostationary communication satellite, with a mass of $2.5 \times 10^3\ \text{kg}$ , orbits the Earth at an altitude of $3.5786 \times 10^7\ \text{m}$ above the Earth's surface.

The mass of the Earth is $5.97 \times 10^{24}\ \text{kg}$ and its radius is $6.371 \times 10^6\ \text{m}$ .

(a) Calculate the magnitude of the gravitational force of attraction between the Earth and the satellite.

[2]

(b)

(b) According to Newton's first law of motion, a non-zero force should cause an object to accelerate. Explain how the force you calculated in (a) causes the satellite to accelerate while maintaining its orbit.

[3]

Question 9

EasyPaper 1A · calculator1 mark

Two satellites, S and T, orbit a planet in circular orbits. Satellite S has mass $m$ and orbital radius $2R$ . Satellite T has mass $4m$ and orbital radius $R$ .

What is the ratio $\frac{\text{centripetal force acting on S}}{\text{centripetal force acting on T}}$ ?

A. $\frac{1}{16}$

B. $\frac{1}{8}$

C. $\frac{1}{2}$

D. $1$

Question 10

MediumPaper 1A · calculator1 mark

A piece of space debris of mass $m$ is in a circular orbit of radius $R$ around Earth. Its orbital speed is $v$ . A second piece of space debris of mass $2m$ is in a circular orbit of radius $4R$ around Earth.

What is the orbital speed of the second piece of debris?

A. $\frac{1}{4}v$

B. $\frac{1}{2}v$

C. $2v$

D. $4v$

Question 11

MediumPaper 1A · calculator1 mark

An exoplanet A has a mass twice that of exoplanet B. The radius of exoplanet A is half that of exoplanet B.

The gravitational field strength at the surface of A is $g$ .

What is the gravitational field strength at the surface of B?

A. $\frac{g}{8}$

B. $\frac{g}{4}$

C. $4g$

D. $8g$

Question 12

MediumPaper 1A · calculator1 mark

What are the fundamental SI units of the universal gravitational constant $G$ ?

A. $\text{kg}^{-1} \text{m}^3 \text{s}^{-2}$

B. $\text{N} \text{m}^2 \text{kg}^{-2}$

C. $\text{kg} \text{m}^3 \text{s}^{-2}$

D. $\text{kg}^{-1} \text{m}^2 \text{s}^{-2}$

Question 13

MediumPaper 1A · calculator1 mark

A research satellite orbits a newly discovered exoplanet. At an altitude where its orbital radius from the center of the exoplanet is $R$ , the gravitational field strength experienced by the satellite is $g_R$ . The satellite then adjusts its orbit to a new altitude where its orbital radius from the center of the exoplanet is $2R$ .

What is the difference in the magnitude of the gravitational field strengths experienced by the satellite between these two orbital radii?

A. $\frac{g_R}{2}$

B. $\frac{g_R}{4}$

C. $\frac{3g_R}{4}$

D. $\frac{g_R}{8}$

Question 14

MediumPaper 1A · calculator1 mark

A space probe is at a distance $r$ from the centre of a planet. The gravitational field strength at this point is $g_0$ . The probe moves to a new position at a distance of $4r$ from the centre of the planet.

What is the magnitude of the change in the gravitational field strength experienced by the probe?

A. $\frac{15}{16}g_0$

B. $\frac{1}{16}g_0$

C. $\frac{3}{4}g_0$

D. $\frac{1}{4}g_0$

Question 15

MediumPaper 1A · calculator1 mark

Two artificial satellites, P and Q, are in circular orbits around a planet. The orbital radius of satellite P is $R$ and the orbital radius of satellite Q is $4R$ . What is the ratio $\frac{\text{orbital period of Q}}{\text{orbital period of P}}$ ?

A. 2

B. 4

C. 8

D. 16

Question 16

MediumPaper 1A · calculator1 mark

Star Alpha has luminosity $L$ and apparent brightness $b$ . Star Beta has luminosity $4L$ and apparent brightness $16b$ .

As viewed from Earth, what is $\frac{\text{parallax angle subtended by Alpha}}{\text{parallax angle subtended by Beta}}$ ?

A. $\frac{1}{4}$

B. $\frac{1}{2}$

C. $2$

D. $4$

Question 17

MediumPaper 1A · calculator1 mark

A comet follows a highly elliptical orbit around a star. Which statement about the comet's motion is incorrect?

A. The speed of the comet is greatest when it is closest to the star.

B. The star is located at one of the foci of the comet's elliptical orbit.

C. The square of the comet's orbital period is proportional to the square of the semi-major axis of its orbit.

D. A line connecting the comet and the star sweeps out equal areas in equal time intervals.

Question 18

MediumPaper 1A · calculator1 mark

Two satellites, Alpha and Beta, orbit the Earth in circular paths.

The average orbital radius of satellite Alpha is four times greater than the average orbital radius of satellite Beta.

What is $\frac{\text{orbital period of satellite Alpha}}{\text{orbital period of satellite Beta}}$ ?

A. $\sqrt{4}$

B. $4^2$

C. $\sqrt{4^3}$

D. $4^3$

Question 19

MediumPaper 1A · calculator1 mark

Two satellites, Alpha and Beta, are in circular orbits around a planet.

The orbital period of satellite Alpha is eight times greater than the orbital period of satellite Beta.

What is $\frac{\text{orbital radius of satellite Alpha}}{\text{orbital radius of satellite Beta}}$ ?

A. 2

B. 4

C. 8

D. 64

Every Gravitational Fields question, marked for you

Every answer is marked mark by mark, IB-style, and the AI tutor helps when you are stuck.

Where marks are lost

Stopping one step short of the conclusion. Two numbers and no sentence is two marks out of three.
Answering a procedure question with a platitude.
Losing precision in Paper 1B. Uniquely to this paper, quoting the right number badly loses marks.

Gravitational Fields: notes and practice questions

How it is examined

Guiding questions

Linking questions

Practice questions

Question 1

Question 2

Question 3

Question 4

Question 5

Question 6

Question 7

Question 8

Question 9

Question 10

Question 11

Question 12

Question 13

Question 14

Question 15

Question 16

Question 17

Question 18

Question 19

Every Gravitational Fields question, marked for you

Where marks are lost

Practise this topic as a session

Questions,
answered.

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bank built for you.

How it is examined

Guiding questions

Linking questions

Practice questions

Question 1

Question 2

Question 3

Question 4

Question 5

Question 6

Question 7

Question 8

Question 9

Question 10

Question 11

Question 12

Question 13

Question 14

Question 15

Question 16

Question 17

Question 18

Question 19

Every Gravitational Fields question, marked for you

Where marks are lost

Questions,answered.

Start with the IB questionbank built for you.

Questions,
answered.

Start with the IB question
bank built for you.