The unit step function is defined as
u
(
t
)
=
1
,
t
≥
0
0
,
t
<
0
u
(
t
)
=
1
,
t
≥
0
0
,
t
<
0
(1)
This function is useful for defining signals which we wish to start at t=0t=0. In other words, often, we would like for signals to be zero for negative values of tt. We can force this situation by simply multiplying by u(t)u(t).