Letter: Weighty matters

Letter:

Weighty matters

Sir: Dr Anthony Egan's letter (3 March) suggests that astronauts are "weightless". If they were they would travel away from the earth in a straight line. It is their weight (somewhat less than their weight at the earth's surface) which provides the centripetal force which keeps them travelling in a circular orbit around the earth. Their spacecraft is behaving in the same way, so in relation to the spacecraft they appear weightless.

His idea that mass is "how much stuff there is" is not wholly incorrect but I think that Newton would have preferred the idea of mass being a measure of the resistance of an object to having its motion changed. Which is why Brussels's use of the word mass is appropriate in the context of bringing a trailer or caravan to a stop in a safe distance.

SEB MOORE

Little Walsingham, Norfolk

London E14