What does Gravitational Fields cover in IB Physics?

This topic extends the study of gravitational fields to include gravitational potential, potential energy, and orbital/escape speeds. Gravitational potential energy for a two-body system is E_p = -G(m_1m_2)/(r), representing work done from infinite separation. Gravitational potential at a point is V_g = -G(M)/(r), defined as the work done per unit mass to bring a mass from infinity to that point.

Is Gravitational Fields SL or HL?

Gravitational Fields is HL only. SL students are not examined on it.

How does FourtyFive help me practise Gravitational Fields?

FourtyFive has 9 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.

Is FourtyFive free for Gravitational Fields practice?

Yes. A free account gives you 50 marked answers a month, and you do not need a card to sign up.

Can I handwrite Gravitational Fields answers on an iPad?

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Topic D.1 · HL only

Gravitational Fields: notes and practice questions

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

Start from the core idea: this topic extends the study of gravitational fields to include gravitational potential, potential energy, and orbital/escape speeds. 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.

5 min read Jump to 9 practice questions

Summary

This topic extends the study of gravitational fields to include gravitational potential, potential energy, and orbital/escape speeds.
Gravitational potential energy for a two-body system is $E_p = -G\frac{m_1m_2}{r}$ , representing work done from infinite separation.
Gravitational potential at a point is $V_g = -G\frac{M}{r}$ , defined as the work done per unit mass to bring a mass from infinity to that point.
Gravitational field strength $g$ is the negative gravitational potential gradient: $g = -\frac{\Delta V_g}{\Delta r}$ .
The work done in moving a mass $m$ in a gravitational field is $W = m\Delta V_g$ .
The escape speed from a point is $v_{esc} = \sqrt{\frac{2GM}{r}}$ .
The orbital speed for a circular orbit is $v_{orbital} = \sqrt{\frac{GM}{r}}$ .
Equipotential surfaces are perpendicular to gravitational field lines.

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 $g = \dfrac{F}{m} = G\dfrac{M}{r^2}$

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

9 questions · 1 easy · 8 medium

Showing 9 of 9

Question 1

EasyPaper 1A · calculator1 mark

Which statement correctly describes one of Kepler's laws of planetary motion?

A. The orbit of a planet is a circle with the Sun at the centre.

B. A line joining a planet and the Sun sweeps out equal areas in equal intervals of time.

C. The orbital period of a planet is directly proportional to the square of its average orbital radius.

D. The gravitational force between a planet and the Sun is constant throughout its orbit.

Question 2

MediumPaper 2 · calculator4 marks

A student is conducting an experiment with a dense metal sphere and a standard physics textbook on a laboratory bench. The metal sphere has a mass of 8.0 kg, and the textbook has a mass of 2.2 kg.

(a) Calculate the approximate gravitational force of attraction between the sphere and the textbook. State any assumptions made.

Question 3

MediumPaper 2 · calculator8 marks

(a)

A robotic probe of mass $3.0 \times 10^3\ \text{kg}$ is in a circular orbit around a newly discovered exoplanet. The orbital radius is $1.2 \times 10^7\ \text{m}$ and the mass of the exoplanet is $3.5 \times 10^{24}\ \text{kg}$ .

(a) Calculate the kinetic energy of the probe.

[2]

(b)

(b) Calculate the gravitational potential energy of the probe.

[2]

(c)

[1]

(d)

(d) Discuss the effect that a small viscous drag force, due to a tenuous exosphere, acting on the probe would have on your answers to (a), (b) and (c).

[3]

Question 4

MediumPaper 2 · calculator10 marks

(a)

This question is about a space probe approaching an exoplanet.

A space probe, with negligible initial kinetic energy, starts from rest at a distance $D$ away from the centre of an exoplanet. The mass of the exoplanet is $M_p$ and its radius is $R_p$ . The gravitational constant is $G$ .

(a) Determine an expression for the speed $v$ at which the probe would impact the exoplanet's surface, in terms of $M_p$ , $G$ , $D$ and $R_p$ .

[3]

(b)

(b) Calculate a value for $v$ for a probe originating with negligible kinetic energy at a very large distance from the exoplanet. The exoplanet's mass is $8.0 \times 10^{24}\ \text{kg}$ and its radius is $7.0 \times 10^6\ \text{m}$ .

[3]

(c)

[2]

(d)

(d) Explain why you might expect your answers to (b) and (c) to be numerically equal.

[2]

Question 5

MediumPaper 1A · calculator1 mark

An exoplanet hunting observatory measures the parallax angle of a newly discovered star, Kepler-186, to be $2.5 \times 10^{-2}$ arc-second.

What is the distance from the Earth to Kepler-186?

A. $0.025 \text{ pc}$

B. $25 \text{ pc}$

C. $40 \text{ pc}$

D. $400 \text{ pc}$

Question 6

MediumPaper 1A · calculator1 mark

Two exoplanets, Alpha and Beta, of identical mass, orbit a distant star. Exoplanet Alpha orbits at a distance $R$ from the star, while exoplanet Beta orbits at a distance $3R$ from the star.

What is $\frac{\text{kinetic energy of Alpha}}{\text{kinetic energy of Beta}}$ ?

A. $1/3$

B. $1$

C. $3$

D. $9$

Question 7

MediumPaper 1A · calculator1 mark

A dwarf planet has a radius $R_0$ . The graph (not shown) illustrates the variation of the gravitational potential due to the dwarf planet with distance $r$ from its centre. At the surface of the dwarf planet ( $r=R_0$ ), the gravitational potential is $-2.5 \times 10^6 \text{ J kg}^{-1}$ .

What is the escape speed from the surface of the dwarf planet?

A. $1.6 \times 10^3 \text{ m s}^{-1}$

B. $2.2 \times 10^3 \text{ m s}^{-1}$

C. $3.2 \times 10^3 \text{ m s}^{-1}$

D. $4.5 \times 10^3 \text{ m s}^{-1}$

Question 8

MediumPaper 2 · calculator13 marks

(a)(i)

A small spherical steel ball is released from rest at the surface of a tall column of glycerin.

The following data are available:

radius of the steel ball = $2.0\text{ mm}$

density of steel = $7850\text{ kg m}^{-3}$

density of glycerin = $1260\text{ kg m}^{-3}$

viscosity of glycerin = $1.4\text{ Pa s}$

acceleration due to gravity $g = 9.81\text{ m s}^{-2}$

(a) (i) Calculate the volume of the steel ball.

[2]

(a)(ii)

(a) (ii) Calculate the initial acceleration of the steel ball.

[4]

(b)

(b) Describe how the acceleration of the steel ball changes as it falls through the glycerin.

[2]

(c)(i)

[2]

(c)(ii)

[3]

Question 9

MediumPaper 1A · calculator1 mark

A spherical planet of radius $R$ has a satellite of mass $m$ that orbits at a height $2R$ above the surface of the planet with kinetic energy $K$ . What is the escape speed from the surface of the planet?

A. $\sqrt{\frac{12K}{m}}$

B. $\sqrt{\frac{8K}{m}}$

C. $\sqrt{\frac{4K}{m}}$

D. $\sqrt{\frac{2K}{m}}$

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

Every Gravitational Fields question, marked for you

Where marks are lost

Practise this topic as a session

Questions,
answered.

Start with the IB question
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

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.