Answer
The net force is zero.
Work Step by Step
$r = (3500 - 160t)~\hat{i} + 2700~\hat{j} + 300~\hat{k}$
$v = \frac{dr}{dt} = (-160~\hat{i})~m/s$
$a = \frac{dv}{dt} = 0$
We can use Newton's Second Law to find the net force:
$F = ma$
$F = (250~kg)(0)$
$F = 0$
Since the acceleration is 0, the net force must also be 0.
