Sir, Anybody disappointed at being prevented from throwing themselves off Magdalen Bridge on May Morning might like to contemplate this A-level physics calculation.

By my measurements, the parapet of the bridge is about 6.2 m (just over 20 ft) above the water, whose depth is a further metre or so, making a total drop H of 7.2 m. If the river were dry, a body falling from the parapet would first touch the riverbed at a speed equal to the square root of 2gH, where g is the acceleration due to gravity; this speed is just under 12 metres per second (27mph). Such a fall would almost certainly lead to serious injury. What of the braking effect of the water? As the water is waist deep, and the density of people is about that of water, it is a reasonable model that a mass of water equivalent to most half of the jumper's mass is displaced on impact.

The displaced water takes momentum from the falling body, slowing it down. A very rough (over)estimate of the momentum transfer is the displaced mass multiplied by the speed at which the body hits the water (11 metres per second), i.e. half the body's momentum on hitting the water. This is enough to slow the body down so that it hits the riverbed at 6.5 metres per second. Thus, the jump into water is at least equivalent to jumping on to solid ground from a wall of 2.2 metres (more than seven feet), a height at which the risk of injury is surely not negligible.

I believe that the calculation above underestimates the impact speed, possibly by a considerable margin; and it is surely an extra risk that the riverbed cannot be seen (imagine jumping off a wall in the dark). My advice is to stay firmly on the pavement.

Sam Howison, Oxford Fair voting needed Sir, Labour city councillors want the city to become a unitary council, because they believe that it is undemocratic for the Conservative-controlled county council to impose its traffic policies on the city (which has no Conservative councillors). It would be simpler to introduce fair voting which would mean that the county council would not have an overall Conservative majority (when only 35 per cent of voters supported the Conservatives) and would also mean that the city council would have a few Conservative councillors.

Dermot Roaf, Oxford