Using differentiation to describe the motion of an object launched upwards. Attachments in PDF format.

Questions are taken from the course book 'Physics with modern physics', Richard Wolfson & J. M. Pasachoff.

A model rocket is launched straight upward; its altitude  as a function of time is given by y = bt-ct^2, where b=68 m/s, c=4.9 m/s, t is the time in seconds, and y is in meters.  

(a) Use differentiation to find a general expression for the rocket's velocity as a function of time.  

(b) When is the rockets velocity zero?