...at greater then the speed of sound in cheese. The leading molecules of cheese cannot transmit the force of impact backward because the speed of force propagation is limited by the speed of sound in cheese so the molecules must move either sideways or into the steel. If the steel is strong enough then the only option is sideways. The area of impact may be large enough that the sideways velocity needs to be larger then the incoming velocity, but this would violate conservation of momentum. I can see the following possible solutions to this dilemma, but I don't know which, if any, are correct. 1. As the cheese hits the steel it compresses, forming a more dense medium through which force can be move rearward. I don't like this idea because it raises the problem of which moves rearward through the cheese first, force or compression. Compressing the cheese requires force, but the force cannot move backward because the cheese is moving forward faster. 2. I cannot treat the steel wall as rigid. It will deform to make room for the compressed cheese and/or cheese molecules will be forced in between metal atoms creating a cheese/steel mixture at the interface. I don't like this idea because replacing the steel with a stronger substance or replacing the cheese with a softer substance returns me to the original problem. Surely there is some combination of substances in which the wall can be treated as rigid and impermeable 3. There are conditions in which the propagation of force through a medium can be faster then the propagation of sound through the same medium. I don't like this idea because the transmission of sound is the transmission of force, it seems self contradictory. So. Is there any truth to any of these? What is really going on at the cheese/steel interface?