the pulse itself. The velocity of the pulses is then  w / t . As always, the velocity of a wave depends on the properties of the medium, in this case the string. The properties of the string can be summarized by two variables: the tension, T , and the mass per unit length, ? (Greek letter mu). If we consider the part of the string encompassed by the initial dent as a single object, then this object has a mass of approximately ? w (mass/length x length=mass). (Here, and throughout the derivation, we assume that h is much less than w , so that we can ignore the fact that this segment of the string has a length slightly greater than w .) Although the downward acceleration of this segment of the string will be neither constant over time nor uniform across the pulse, we will pretend that it is constant for the sake of our simple estimate. Roughly speaking, the time interval between n /1 and n /2 is the amount of time required for the initial dent to accelerate from rest and reach its normal, flattened position. Of course the tip of the triangle has a longer distance to travel than the edges, but again we ignore the complications and simply assume that the segment as a whole must travel a distance h . Indeed, it might seem surprising that the triangle would so neatly spring back to a perfectly flat shape. It is an experimental fact