D1

• force between two masses is proportional to the product of the two masses and inversely proportional to the square of the distance between the mass
• Newton's universal law of gravitation


  an attractive force exists between any two point masses that is proportional to the product of the masses and inversely proportional to the square of their separation. This force is directed along a line joining the two masses
• point mass


  a theoretical object of no size having mass
• gravitational field


  a region of space in which a mass experiences a gravitational force
• gravitational field strength


  the gravitational force per unit mass exerted on a point mass
• uniform gravitational field


  a gravitational field in which the field strength is constant at all locations
• gravitational field lines


  lines that show the direction of the force exerted of the ellipse
• Kepler's first law


  planets move on ellipses with the Sun at one of the foci of the ellipse
• Kepler's second law


  the line joining a planet and the Sun sweeps equal areas in equal times
• Kepler's third law


  The period of revolution of a planet around the Sun is proportional to the 3/2 power of th semi-major axis of the ellipse
• geostationary orbit


  a satellite's orbit around the Earth, above the equator, that has an orbital period of exactly one day so that the satellite remains above the same spot on the Earth's surface
• escape speed 


  the minimum speed of an object in a gravitational field so that the object reaches infinity with zero kinetic energy
• equipotential surfaces


  a surface with a constant value of the potential at all points on the surface