Saturday, December 11, 2010

General Physics Exam

If the law were changed so that traffic in Great Britain travelled on the right-hand side of the road instead of on the left, would the length of the day increase, decrease or be unaltered? Explain the reasoning behind your answer.

It would be unaltered.

Driving in the opposite direction to the Earth's rotation may have some effect due to the conservation of angular momentum, but roads in Britain go in many directions, so the effect would cancel out.

When the driving in the opposite direction to the Earth's rotation, angular momentum is required, so when the car starts a small fraction is "robbed" from the planet. However, this will be returned as soon as the car stops - in fact, it is necessary if the car is to stop thanks to the conservation of angular momentum. Friction takes care of the stopping (and hence the return of momentum) for you.

Even if we assume that people drive in the opposite direction, and do not stop, sooner or later they are going to give in and come home. Probably they will do this before completing half a circuit of the Earth's circumference (after all, we're only looking at Britain), so it will make the most sense to come back the way they came. Travelling in the opposite direction will also return the momentum to the Earth.

If we ignore all this, and pretend that either stopping, returning, or other cars moving in other directions does not lead to a cancelling out of the removal of angular momentum, we still encounter a problem. The Earth is huge, your car is not. The effect of a car's momentum being gained from the Earth would be so minor that it would likely not be measurable, since the mass of the Earth is so much greater. So, since the difference between the mass of the car and the mass of the Earth is so great, the change in the length of the day would be negligible.

That is, unless your mum is driving the car. Fortunately, she's too stupid to drive.

I thank you.


  1. Sean! A blogger account too? It seems you are all over the internet. Perhaps you have a blog that I can read?