Why are four-person bobsleighs faster than two-person bobsleighs? Is it because they have two extra people pushing at the start and therefore are faster throughout their run or is it to do with weight and momentum? Surely there comes a point when the extra weight slows the bob too much? A few friends and I spent an hour or so arguing over this the other week, divided into those who thought it was the extra weight and others who thought it was the extra runners.

Alan Kane, Glasgow, UK

Is this true no matter how steep the hill is? Which uses more calories?

After a long journey, it seems to me - and I've heard other people say it - that cars are less besmattered with flies than they used to be maybe 30 years ago.

Is this the case? Has anyone measured it?

If it is true, is it because:

1. There are fewer insects?
2. There are more cars, so they get fewer flies per car?
3. Unnatural selection has favoured high-fliers?

The photograph (right) was taken near Maldon, Essex, in the UK, looking directly overhead. It appears to show the result of an aircraft flying through thin cloud and dispersing it along its flight path. If an aircraft was responsible it had long since passed when the picture was taken. Is this a common sight and what mix of conditions is required to produce the effect?

Why can't you mix cross-ply and radial tyres on a car?

Passengers inside an aeroplane, that is going through a free fall can feel the effect and if the fall is fast enough, they could even be levitated inside the plane. The same effect goes with a free falling elevator.

But passengers inside a bullet train feels no significant changes even though the speed of a bullet train is faster than that of a free-falling elevator.

Is there any explanation for this?

We were taught in school that aeroplanes fly because the wings are bulbous on top, so the air has further to travel, therefore does it faster, therefore at a lower pressure, so developing lift. I heard this explanation given by a teacher to a class at an 'hands on' demonstration in the Science Museum with an expanded polystyrene wing and a hair dryer - shure enough the wing convincingly shot upward when inserted in the air flow. After they had gone I tried it myself, and it became clear to me that the convincing lift came not from the Bernoulli effect but from the angle of attack - held level there was no detectable lift; the children were being conned.

And in any case, aeroplanes can fly upside down, can't they?

So for a Jumbo at cruising altitude in level flight what proportion of the lift is due to the angle of attack, and what proportion to the hump in the aerofoil?

When the location of a place along a road is specified by a distance (for example 20 km off a town) how this is mentioned in a mailing address?

A popular UK car show demonstrated that, if you own an automatic, electronic car key, it is possible to lock and unlock your vehicle from a greater distance if you point the key to your head as you press the button. Why is this?