Answer
The reading on the scale is 922 N.
Work Step by Step
$v(t) = (3.0~m/s^2)~t + (0.20~m/s^3)~t^2$
$a(t) = \frac{dv}{dt} = (3.0~m/s^2) + (0.40~m/s^3)~t$
At t = 4.0 s:
$a = (3.0~m/s^2) + (0.40~m/s^3)(4.0~s)$
$a = 4.6~m/s^2$
We can find the normal force $F_N$ exerted on the person. Note that the normal force is equal to the reading on the scale.
$\sum F = ma$
$F_N - mg = ma$
$F_N = ma + mg$
$F_N = (64~kg)(4.6~m/s^2)+(64~kg)(9.80~m/s^2)$
$F_N = 922~N$
The reading on the scale is 922 N.
