Answer
(a) $5.25Kg$
(b) $4.66Kg$
Work Step by Step
(a) We know that
$F=mg$
$\implies m=\frac{F}{g}$
We plug in the known values to obtain:
$m=\frac{51.5N}{9.81m/s}$
$m=5.25Kg$
(b) We know that in the given scenario
$F-mg=ma$
$\implies F=m(a+g)$
This can be rearranged as:
$m=\frac{F}{a+g}$
We plug in the known values to obtain:
$m=\frac{51.5N}{1.25m/s^2+9.81m/s^2}$
$m=4.66Kg$
