Help Intro Physic problem dealing with Kinetic

[Kennyh]

Homework Statement

A model rocket is launched straight upward with an initial speed of 46.7 m/s. It accelerates with a constant upward acceleration of 2.16 m/s2 until its engines stop at an altitude of 138 m.
What is the maximum height reached by the rocket?

How long after lift off does the rocket reach its maximum height?

How long is the rocket in the air?

Homework Equations

What I found out from the given information above is that xi=o,xf=138m,vi=0m/s,vf=?,a= 2.16m/s2, and t= is ?.
I used this equation = vf^2=2a(xf-xi)+vi^2 to find Vf and vf was 52.7 m/s. from this point on I do not know how to find the maximum height or the other two question? Please help thank you very much

The Attempt at a Solution

 

[Quincy]

From the point where the engine stops, its acceleration becomes -9.8 m/s^2, so treat it as a free fall problem, in which Vi = 52.7 m/s, Xo = 0, Xf = ?, Vf = 0, a = -9.8 m/s^2, solve for Xf, and then add that height to 138, and that would be the maximum height.

 

[rl.bhat]

In the given problem, the initial velocity vi is not zero. It is 46.7 m/s .

 

[Quincy]

In the given problem, the initial velocity vi is not zero. It is 46.7 m/s .

He probably made a typing error, he got Vf for the first part correct...

 

[Quincy]

thanks a lot Quincy, as for the informations you gave me I was looking at this equation --- xf = Vi(t) + (1/2)(at^2) - Xi. Do I have to find an equation to figure out time first? thanks sorry for the inconveniences.

Yes, figure out the time first, using the equation Vf = Vi + at

 

[Kennyh]

I found out t= 5.38s
So I plug it in the equation: Xf = 52.7*5.38+(1/2)(-9.8)(5.38)^2 = 141.7 m
Then 138+141.7 = 279.7m but it was not the right answer, am I doing this wrong?

 

[Quincy]

I found out t= 5.38s
So I plug it in the equation: Xf = 52.7*5.38+(1/2)(-9.8)(5.38)^2 = 141.7 m
Then 138+141.7 = 279.7m but it was not the right answer, am I doing this wrong?

What does your book say is the right answer?

 

[Kennyh]

It is not the book, it's an online homework

 

[Quincy]

It is not the book, it's an online homework

Perhaps it's a rounding error; does it specify to round to the nearest tenth/hundredth/etc?

 

[Kennyh]

I don't think so the computer is very easy on those if you a couple off the decimal it is fine. thanks for your help

 

[Quincy]

Another user (malik123) was having the some problem in a similar thread; maybe you guys are on the same online program?