What feature of a planet deteremines the speed of a satellite orbiting it?

Question

We have been asked to explain this in Physics terms (must hacve at least 2 ideas linked together)

Answer 1

Bruce is right, but a little vague as Kepler's third law only works of the sun and a planet.
The answer you are looking for is mass and distance. You can see them in Newton's adaptation of Kepler's third law. The more mass an object has, the stronger the gravity is has. On the other hand, the farther away from the mass you get, the less gravity you feel. (gravity grows weaker with the square of the distance)
So the speed of a satellite is a balance between the mass of the planet and the distance of the satellite. If the satellite is closer to the planet or if the planet has more mass, the satellite must move faster to maintain its orbit. If the planet has less mass or if the satellite is farther away, it need not move as fast.

Answer 2

Keplets Laws of Planetary Motion, in particular the 3rd one explain how the time taken to make an orbit is mathematically related to the average distance the satellite is from the planet.