Last week I played soccer for the first time in years. The goalkeeper kicked the ball high (probably about 20 metres into the air) and a teammate called to me to head it. I realise I am lacking in skill, but the power of the ball striking my head knocked me off my feet, bruised my forehead and left me with a dreadful headache. Yet professional footballers seem able to head higher and faster-moving balls with no apparent damage or pain. What velocities and forces are they dealing with, and why did the ball leave me stunned but not a professional player?
Alan Nicholas, Barrow-in-Furness, Cumbria, UK