A construction worker accidently drops a hammer from a height of 90 m while working
on the roof of a new apartment building. The height, s, in metres, of the hammer after t
seconds can be modelled by the function s(t)=90-4.9t^2, t>0.

a. Determine the velocity at 1 s and 4 s.

b. When will the hammer hit the ground?

c. Determine the impact velocity of the hammer.
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