Putting a brake on friction theory

rom Dr Ian Craighead

Sir: It is commendable that your readers are keen to help British Rail overcome the problem of stopping trains in the presence of leaf mould on the track (Letters, 23 February).

Unfortunately, however well versed your correspondent, Mr Horncastle, is in third-form geometry, he seems to have forgotten his third-form physics. A fundamental law of friction tells us that the frictional force which is generated when two surfaces come into contact is independent of the area of contact. The improvement claimed for continental trams is probably largely due to the track being continually "scrubbed" by rubber-tyred vehicles, which doesn't happen too often on BR tracks.

Yours faithfully,

IAN CRAIGHEAD

University of Strathclyde

Glasgow

23 February