A construction worker accidentally drops a hammer from a height of 90m while working on the roof of an apartment building. The height, s, in meters, of the hammer after t seconds can be modeled by the function

s(t)=90 - 4.9t², t > 0

a. determine the velocity of the hammer at 1 s and 4 s

b. when will the hammer hit the ground?

c. Determine the impact velocity of the hammer.