Gravitational field
Gravitational Fields - OCR A-Level Physics
Key Definition
Gravitational field
A region of space in which a mass experiences a force due to the gravitational attraction of another mass.
A region of space in which a mass experiences a force due to the gravitational attraction of another mass.
Key Definition
Gravitational field strength (g)
The gravitational force per unit mass at a point in the field. Units: $N kg^{-1}$ (equivalent to $m s^{-2}$).
The gravitational force per unit mass at a point in the field. Units: $N kg^{-1}$ (equivalent to $m s^{-2}$).
- $g = F/m$, where F is the gravitational force on a test mass m placed at the point.
- Near Earth's surface, $g \approx 9.81$ $N kg^{-1}$ (approximately uniform).
- Field linesLines showing the direction of the gravitational force on a test mass. They always point towards the attracting mass. point towards the centre of the mass (gravity is always attractive).
- For a uniform field (near Earth's surface): field lines are parallel and equally spaced.
- For a radial field (around a planet): field lines radiate inward, getting closer together near the surface.
- g is a vectorA quantity with both magnitude and direction (e.g. force, velocity, displacement). quantity - it has both magnitude and direction (towards the mass).
Examiner Tips and Tricks
- g has two roles: gravitational field strengthThe gravitational force per unit mass at a point in a gravitational fieldA region of space in which a mass experiences a gravitational force.. Measured in N kg⁻¹. ($N kg^{-1}$) and accelerationThe rate of change of velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹.. A vector quantity. Measured in m s⁻². due to gravity ($m s^{-2}$).
- These are numerically identical but conceptually different.
- In field questions, use $N kg^{-1}$.