Astronauts in orbit experience weightlessness. They are in free fall in the Earth's gravity field, just as are the craft in which they live. Inside the craft, they are free to float around subject to no force RELATIVE to the craft. This can make everyday activities difficult. Astronauts cannot drink a normal cup of tea or use a toilet in the normal way. Food will not stay on a plate, and astronauts cannot use their muscles in a natural way, which means that over a period of time, their muscles waste away. Artificial gravity would be extremely useful on a spacecraft in orbit.We cannot make gravity artificially in the way the writes of Star Trek would have us believe, but we can create forces to mimic the effect of gravity by setting a spacecraft spinning.

The centripetal force is given bywhereis the radial distance of the object from the hub of the spacestation,is the mass of the object andis the speed of rotation of the object. To mimic gravity equal to gravity on Earth we must have

This corresponds to a period of