Gerald Cunningham of Lauder asks :-

Why can a blowfly fly full speed into a glass window without appearing to suffer any ill-effects?

David Logan, a physicist at Monash University's Accident Research Centre, responded.

Let us investigate the simple physics of the 'crash' event. A blowfly has a mass of around 80-100 mg and can fly at up to 2.5 metres per second. Let us propose that a 5mm length blowfly stops from full speed in 1mm when it hits the windowpane, mainly through body deflection, since we can assume the window is effectively rigid to a fly.

To determine the the deceleration of the fly we can use the standard kinematic equation known to high-school physics students, v times v = u times u + 2ad. (Note that this is just a statement of the conservation of energy. Add an m divided by 2 to each term to show that this equation just states that the change in kinetic energy is equal to the work done on the fly, ie force times distance.) In our case v, the final velocity, = 0 metres per second; u, the initial velocity = 2.5 metres per second; d (the distance the deceleration takes place over) = 0.001 m. Solving for a, the deceleration or negative acceleration, we can calculate that the deceleration of the blowfly during this event is 3125 metres per second squared. For comparison, the acceleration due to gravity at the Earth's surface is nearly 10 metres per second squared! So our fly decelerates at more than 300 times the acceleration due to gravity, a very high ratio.

However, to put this into perspective, the force on the blowfly (force is mass times acceleration) to produce this deceleration is just 0.3 Newtons (given its mass of 100 mg or 0.0001 kg). This is a tiny force, equivalent to the gravitational force acting on a 30 mg mass, and is potentially unlikely to damage the insect.

This results from the fact that volume (and therefore mass) increases with the cube of the size of an object. For example, an object that is half the diameter of another will have only one eighth the volume and mass. Given the same acceleration, mass is what affects the force required to decelerate two different objects and explains why a much larger object, like a human, undergoing a similar event would be much more severely injured! Even allowing for the fact that with a body as massive as a human, the glass would also flex giving a longer distance before the body comes to a halt (or the glass breaks, absorbing some of the energy).