 | Categories | | Nooks and crannies |  | | Yesteryear | | | Semantic enigmas | | | The body beautiful | | | Red tape, white lies | | | Speculative science | | | This sceptred isle | | | Root of all evil | | | Ethical conundrums | | | This sporting life | | | Stage and screen | | | Birds and the bees | | | SPECULATIVE SCIENCEHow much does the Moon weigh? - THE head of physics at Isaac Newton's old school he may be, but Mr Bomphrey is surely talking nonsense. Is he not confusing weight and gravity? Gravitational force varies as the inverse square of distance as he rightly says, but what can he mean by saying that the experience of an astronaut 'is not true weightlessness but he feels weightless'? He feels weightless because he is weightless (ignoring the minute attraction to his spacecraft). No experiment he did (short of actually looking out of the window), would enable him to tell that his spacecraft was held, by gravitational force, in orbit around the Earth. Weight is not the force of gravitational attraction, but describes how an object resists gravity. The whole point about orbital free-fall is that the object is not resisting gravity, but is continually accelerating under its influence (the acceleration causing orbital motion, of course, rather than a linear increase in velocity). To use Mr Bomphrey's established analogy, if I pick up an apple I need to apply a force of one newton to hold it up. An astronaut does not need to apply this force, so the apple is weightless. He still needs to apply a force to move it, as it does, of course, continue to have mass. Mr Bomphrey rightly praises the SI system of units which correctly distinguishes between weight and mass. In the Imperial system, with the two different quantities having the same name, there was endless scope for confusion, and for the inventing of alternative obscure units. Whether the Moon is weightless is quite a difficult, and pointless, question to answer. All parts of the Moon are not in free-fall around the Earth, because they are constrained in their orbit by the shape of the Moon. The nearer bits of Moon are moving too slowly for free-fall; the further bits too fast.
David Gibson, Leeds. - THERE is something about physics that makes arguers quite insultingly dismissive of other people's opinions. Mr Bomphrey and Mr Gibson are using different definitions of the word 'weight'; both of them argue correctly from these different definitions. Mr Bomphrey is using the current textbook and physics syllabus definition of 'weight', which says: 'The force of gravity on an object is called its weight' ( GCSE Steps in Physics by Byron). He is therefore correct to suggest that weightlessness will occur only in situations where the object experiences no gravitational attraction towards other objects. Mr Gibson uses the older definition of weight as described in Ordinary Level Physics by Abbott: 'The weight of a body is the force it exerts on anything which freely supports it.' The book goes on to suggest that the body will usually exert this force due to its being attracted towards the centre of the Earth by gravity. This definition makes Mr Gibson's analysis of the situation correct - although suggesting that weight is 'how an object resists gravity' does tend to give the responsibility for weight to the object rather than to that which supports it. The difference in the two views is possibly due to the attempts made over the last 10 years to make physics accessible to a wider proportion of the school population, which has resulted in a simplification of the ideas taught and a significant decrease in the content, which has left Abbott's 'O-level' textbook as a good introduction at A-level to many topics.
Jonathan Parkinson, Head of Physics, St Thomas More School, Blaydon, Northumberland. Add your answer  |
