Gravitational Fields

• What is a gravitational field?

  A region in which a mass will experience a gravitational force.
• What is gravitational field strength?

  The force per unit mass exerted on test mass at that point in the field.
• What are the units of gravitational field strength?

  N kg⁻¹
• What does the direction of a gravitational field line show?

  The direction of force on a test mass at that point in the field.
• What is represented by the spacing of field lines in a field line diagram?

  Field strength.
• What is a radial gravitational field?


  A gravitational field produced by a spherical (or point) mass in which the field lines all point towards the centre of mass of the object causing the field.
• What is a uniform gravitational field?

  A region in which the gravitational field strength is constant.
• How is a uniform gravitational field shown in a field line diagram?

  Parallel, equally spaced field lines.
• What is a point mass?

  A idealised mass with zero size. The field outside a spherical mass can be modelled as the field around a point mass.
• State Newton's Law of Gravitation.

  The gravitational attraction between two objects is proportional to the product of their masses and inversely proportional to the sqaure of the distance between their centres of mass
• State Kepler's 1st Law.


  A planet's orbit is an ellipse with the Sun at one of the two foci
• What is an aphelion?

  The point in an orbit when an object is furthest from the sun and travelling slowest.
• What is a perihelion?

  The point in an orbit when an object is closest to the sun and travelling fastest.
• State Kepler's 2nd Law.


  A line segment joining the planet to the Sun sweeps out equal areas in equal time intervals.
• State Kepler's 3rd Law.


  For planets in the solar system the square of the orbital period is proportional to the cube of the average orbital radius.
• How does graviational field strength vary with distance from a point mass?

  It follows an inverse square law.
• What is gesostationary satellite?


  A satellite with an orbital period of 24 hours which orbits above the equator in the same direction as the Earth's rotation so its apparent position in the sky remains unchanged.
• What is a satellite?

  An object which orbits planet.
• What is the absolute gravitational potential of a point?

  The work done per unit mass to move an object from an infinite distance away to that point in the field.
• How does graviational potential vary with distance from a point mass?

  It is inversely proportional to distance.
• What is escape velocity?

  The velocity at which the magnitude of an object's kinetic energy is larger than the magnitude of its gravitational potential energy.
• What is gravitational potential energy?

  The work done to move an object from infinity to a point in the gravitational field.