Kirk Sidon, of Ardgowan School, asks :-

The pupils of Room 9, at Addington Primary School, asked:-

What is gravity?

Noel Doughty, an astrophysicist at the University of Canterbury, responded.

In describing things in terms of more basic things, gravity is near the starting point of the chain. Only space and time are more fundamental. This makes gravity more difficult to understand than many other concepts.

Gravity is an attractive force acting between all pairs of objects. Another fundamental example is the electrical force but it only acts between charged objects. Charges can both attract and repel as when small bits of paper are attracted to (or repelled from) a comb that has been charged by using it on one's hair a few times. But gravity always attracts so it keeps growing as we consider heavier objects.

Hence heavy objects cause strong gravitational forces such as the earth's downward attraction on us. The sun's gravitational attraction keeps all the planets in orbit. The gravitational attraction between people is negligible. We are held near the earth's surface by a balance between the earth's gravitational attraction and the electrical repulsion of the surface on our shoes. If we rise above it we soon fall back.

During a fall, we don't feel any gravitational effects at all until we hit the surface. Gravity just accelerates us down but the much faster deceleration caused later by the surface can do us considerable harm.

A spaceship placed into a circular orbit is effectively in zero gravity. Gravity pulls it to the earth's centre but the spaceship's inertia tries to keep it moving in a straight line. The two effects exactly cancel. Its as if the spaceship was continually 'falling' in a circle toward the attracting earth.