OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
(c) gravitational field lines to map gravitational fields
Full specification wording
Point and spherical masses: (c) gravitational field lines to map gravitational fields
Gravitational field lines are a visual tool to map the gravitational field around a mass.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 3 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Gravitational field lines are a visual tool to map the gravitational field around a mass. For a point mass or a uniform sphere, the field is radial: lines point towards the centre of the mass, showing the direction of the gravitational force on a small test mass. The spacing of lines indicates field strength: closer lines mean a stronger field. Around a single spherical mass, lines are evenly spaced in angle but their density decreases with distance because field strength follows an inverse-square law. For two masses, the pattern shows attraction, with lines converging on each mass. Field lines never cross, as the field direction is unique at any point. They are used to represent the field without calculations, helping to predict the motion of objects.
What earns the marks
- Field lines point towards the centre of a spherical mass, indicating attraction.
- The direction of the field line at a point gives the direction of the gravitational force on a small test mass placed there.
- The spacing of field lines represents the relative strength of the gravitational field: closer lines indicate a stronger field.
- For a point mass or uniform sphere, the field is radial and the line density decreases with distance from the centre.
- Field lines do not cross because the gravitational field has a unique direction at each point.
Common mistakes and how to put them right
Misunderstanding: field lines point away from a mass.
Put it right: Gravitational field lines point towards the mass because gravity is always attractive.
Misunderstanding: field lines are equally spaced everywhere around a spherical mass.
Put it right: Spacing increases with distance from the mass, showing that field strength decreases.
Misunderstanding: field lines can cross.
Put it right: Field lines never cross because that would imply two different force directions at the same point.
Misunderstanding: field lines represent the path of an object.
Put it right: They show the direction of the gravitational force at a point, not the trajectory of a moving mass.
Exam tips
- Remember that gravitational field lines always point towards the mass; use arrows to show direction.
- Use the spacing of lines to compare field strengths: denser lines mean a stronger field.
- Check that lines are radial for a single point mass or uniform sphere and do not cross.
Try the lesson
The start of the Point and spherical masses: (c) gravitational field lines to map gravitational fields lesson, made from checked teaching notes for this specification point.
Listen
Gravitational field lines are a visual tool to map the gravitational field around a mass. For a point mass or a uniform sphere, the field is radial: lines point towards the centre of the mass, showing the direction of the gravitational force on a small test mass. The spacing of lines indicates field strength: closer lines mean a stronger field. Around a single spherical mass, lines are evenly spaced in angle but their density decreases with distance because field strength follows an inverse-square law. For two masses, the pattern shows attraction, with lines converging on each mass. Field lines never cross, as the field direction is unique at any point. They are used to represent the field without calculations, helping to predict the motion of objects. A strong answer earns marks for these points. Field lines point towards the centre of a spherical mass, indicating attraction. The direction of the field line at a point gives the direction of the gravitational force on a small test mass placed there. The spacing of field lines represents the relative strength of the gravitational field: closer lines indicate a stronger field. For a point mass or uniform sphere, the field is radial and the line density decreases with distance from the centre. A common slip: Misunderstanding: field lines point away from a mass. Instead, gravitational field lines point towards the mass because gravity is always attractive. Exam tip: Remember that gravitational field lines always point towards the mass; use arrows to show direction.
Key words
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Gravitational field lines
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Radial field patterns
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Field strength and line spacing
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Direction of gravitational force
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See every definition
- Gravitational field lines
- A visual tool to map the gravitational field around a mass, used to represent the field without calculations.
- Radial field patterns
- For a point mass or a uniform sphere, the field is radial, with lines pointing towards the centre of the mass.
- Field strength and line spacing
- The spacing of lines indicates field strength: closer lines mean a stronger field, and line density decreases with distance from the centre.
- Direction of gravitational force
- The direction of the field line at a point gives the direction of the gravitational force on a small test mass placed there.
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