Find displacement, time and velocity

[kari82]

A rocket takes off vertically from the launchpad with no initial velocity but a constant upward acceleration of 2.25 m/s2. At 15.4 s after blastoff, the engines fail completely so the only force on the rocket from then on is the pull of gravity.
(a) What is the maximum height the rocket will reach above the launchpad?
(b) How fast is the rocket moving at the instant before it crashes onto the launchpad?
(c) How longer after engine failure does it take for the rocket to crash onto the launchpad?


a)
deltay=vi(t)+1/2(a)(t)^2
deltay=0(15.4)+1/2(2.25m/s^2)(15.4s^2)
deltay=266.8m

b)Vf^2=vi^2+2(g)(deltay)
vf=+/-squareroot(2(-9.8m/s^2)(-266.8m))
vf=-72.3m/s

Can anyone tell me if what I'm doing is correct? and can i get a hint on how to solve question (c)?

Thanks!

 

[Ry122]

v= u + at
and you said that the maximum velocity the rocket reaches is 72.3ms
the rocket from it's max height is accelerating towards Earth at a speed of 9.81ms.

there's your two values. just plug them into the equation.

 

[kari82]

Thank you!