Hi there, my wife just asked me how hard the impact is when two sides of a rugby scrum come together. I think she's looking for a "like when a truck hits a slightly smaller truck" answer, but now I'm really interested. We're watching Japan v New Zealand at the world cup and the two scrum weights are 850kg and 910kg respectively. They stand a metre apart (at least according to the rules) and so each pack accelerates over half a metre then come to a relatively complete stop on impact. I understand F=ma and the conservation of momentum, but I trip up when I try to work out the rate of acceleration and the final speed reached at point of impact.