Motion in One Dimension: Model Rocket

[jojo711]

Homework Statement

A model rocket accelerates upward from the ground with a constant acceleration, reaching a height of 76 m in 7.2 s.
What is the acceleration (in m/s 2)?
What is the speed (in m/s) at a height of 76 m?

Homework Equations

V=Vo+at
X=Xo+Vot+1/2at^2
V^2=Vo^2+2at(X-Xo)

The Attempt at a Solution

I tried 76=1/2a(7.2)--> 76=3.6a-->21.1=a
but it was wrong. Please help me!

 

[grzz]

My calculator shows 2.93...

 

[jojo711]

Thank You! It said it was right for the acceleration, but could you possibly tell me how you got the answer? And, did you attempt the second part of the problem at all?

 

[grzz]

7.2 must be squared!

 

[jojo711]

So, I would do 2.93(7.2^2)?
I got 151.8912 as my answer...

 

[jojo711]

That's not right... :(

 

[grzz]

i was pointing out your mistake for the acceleration.

 

[jojo711]

oh, well. thank you. Do you know how to do the second part of the problem by any chance?

 

[grzz]

Is the rocket moving vertically upwards?

 

[jojo711]

Yes.

 

[grzz]

Try to use conservation of energy.

 

[jojo711]

Could you explain what you mean by that? I have no idea what that is.

 

[grzz]

What kind of energy does it have initially?
What kind of energy does it have at 76m?

 

[jiboom]

you know it takes 7.2 seconds to reach 76m
initially velocity was 0
and you have worked out the acceleration in part 1
looks like you need

v=u+at
or

v^2=u^2+2as

 

[jojo711]

it's not moving, right...?

 

[jiboom]

i don't know. have you used the suvat equations and worked out the value for the speed?

 

[jojo711]

I did it! You guys are sooo helpful! Thank you so so so so so much!

 

[jiboom]

i feel your pain getting to grips with mechanics :)

i am so bad at this stuff, but find it hard to get any help. i post all over the net to no avail :(

 

[jojo711]

same here! this website pretty much saved my life!