26 September, 2010

Flies hit by trains

I was wondering about this, and I think I've explained it satisfactorily:

Consider a fly getting hit by a train. The fly is initially hovering at rest relative to the ground. At the instant when the train hits it, they have the same speed. Does that mean the train stops momentarily? Does the fly stop the train?

Situation 1:
The fly is a point with small mass.
First let me explain this: things must bend (strain) to apply a force. Before you put a book on a table, does the table apply any force? No. After you place the book, the table applies a force to equalise the book's weight, keeping it stationary. Where does this force come from? The table deflects slightly, so slightly that you can't see it (unless you have a lousy table).
At the instant of impact, the windscreen of the train bends slightly. Glass is very stiff, so this slight bending imparts a very large force on the fly. The large force results in a large acceleration. Together, the bending of the glass and the acceleration of the fly result in them moving together.
Don't understand? Imagine the windscreen of the train is a rubber sheet. When a fly hits it, the rubber sheet bends inwards, towards the train. The part of the sheet that bends actually slows down. At the same time, the fly speeds up. The fly and sheet remain in contact.

Situation 2:
Point heavy mass e.g. pebble.
This mass requires a larger force for the same acceleration, so the glass must bend more. If the glass bends too much, it cracks. This is what happens if a large enough stone hits the windscreen.

Situation 3:
Soft heavy mass e.g. tomato.
This time, different parts of the soft mass can accelerate at different rates. When the train hits it, the outer layers accelerate first, then the inner layers. This inconsistent acceleration is what causes damage to the object.

It may seem trivial, but I think these thought experiments are important in the consistency of physics.

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