Speed of waves in a cup of coffee that spills over, help please!! "When you walk with a cup of coffee (diameter 9 cm) at just the right pace of about 1 step per second, the coffee builds up its "sloshing" until eventually, after a few steps, it starts to spill over the top. What is the speed of the waves in the coffee?" I would show my work but I have NO clue how to even approach this. Only thing I can think of to use is the longitudinal wave speed in a fluid which is v = square root ( bulk modulus / density). But I don't know density of coffee or how to calculate bulk modulus. Can someone point me in the right direction? Thanks.