John Holmes, of Eastbourne, asks :-

Why do flies appear to suffer no damage when they bash into window panes?

John Campbell, a physicist at the University of Canterbury, responded.

Your question involves a common observation. We regularly see flies hit windows without seemingly being hurt. However, we also know from our general observation that a fly hitting the windscreen of a fast car is splattered. Somewhere in-between is the critical "safe" speed.

The answer to your question is because the force on the fly during this collision of a light object travelling at slow speed into a solid object isn't very high.

We can quantify this. A blowfly has a mass of about 100 mg, is about 5 mm long and flies at up to 2.5 metres per second. It will be reasonable to assume a 5 mm long blowfly stops from full speed in 1 mm when it hits the windowpane, mainly through body compression and deflection. (We can assume the window is effectively rigid to a fly.) Any physics student who knows the kinematic equation can deduce, as did David Logan, a physicist at Monash University's Accident Research Centre who has to carry out related calculations for any crash, that the fly decelerates at some 300 times the acceleration due to gravity. This seems incredibly high but the force on the fly (mass times acceleration) is only 0.0001kg times 300 times 10 metres per second per second equals 0.3 Newtons, a tiny force equal to that due to gravity on a mass of 30 mg.

If we go to slightly larger masses, we observe that small birds also usually fly away after flying into a window pane, but not after hitting a medium speed car. Slightly larger birds get stunned or break their necks on hitting a window pane. This applies to fledgling thrushes. The older birds have normally learned not to fly into window panes.

Larger animals (humans, sheep) can run into window panes at low speeds without serious damage because both the window pane and their bodies flex. But if a sheep is hit by a car travelling at a reasonable speed they will be badly damaged if not killed. An animal with a higher centre of gravity, such as a horse, cow, deer or human will likely go through the windscreen, smashing it and themselves and, as likely as not, killing or badly injuring the car driver or passenger.