A transfer function can be represented as a block diagram (Figure
2.21) with X(s) the input, Y(s) the output and the transfer function G(s)
as the operator in the box that converts the input to the output. The block
represents a multiplication for the input. Thus, by using the Laplace
transform of inputs and outputs, we can use the transfer function as a
simple multiplication factor, like the gain discussed in Section 2.2.