Answer
(a) $5.38s$
(b) The answer doesn't depend on the mass.
(c) $9.8m/s^2$ upward.
Work Step by Step
(a) We can find the required time as follows:
$v=\sqrt{rg}$
$\implies v=\sqrt{(7.2m)(9.8m/s^2)}=8.4m/s$
Now $t=\frac{2\pi r}{v}$
We plug in the known values to obtain:
$t=\frac{2(3.1416)(7.2m)}{8.4m/s}$
$t=5.38s$
(b) We can see that the velocity of the wheel does not depend on the mass and consequently the time required to complete the revolution does not depend on the mass of the wheel as well.
(c) We know that
$a_c=\frac{v^2}{r}$
We plug in the known values to obtain:
$a_c=\frac{(8.4m/s)^2}{7.2m}=9.8m/s^2$
Thus, this acceleration is equal to that of gravity and it is directed upward.
