If a stone is thrown vertically upward from the surface of the moon with a velocity of 10 m/s, its height (in meters) after $t$ seconds is $h(t)= 10t-0.83t^2$.
(a) What is the velocity of the stone after 2 seconds?
(b) What is the velocity of the stone after it has risen 25 m?

(a) $v(2)=$ m/s
(b) $v=$ m/s