# Homework Help: Motion in One Dimension: Model Rocket

1. Oct 19, 2011

### jojo711

1. The problem statement, all variables and given/known data
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?

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

3. The attempt at a solution
I tried 76=1/2a(7.2)--> 76=3.6a-->21.1=a

2. Oct 19, 2011

### grzz

My calculator shows 2.93...

3. Oct 19, 2011

### 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?

4. Oct 19, 2011

### grzz

7.2 must be squared!

5. Oct 19, 2011

### jojo711

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

6. Oct 19, 2011

### jojo711

That's not right... :(

7. Oct 19, 2011

### grzz

i was pointing out your mistake for the acceleration.

8. Oct 19, 2011

### jojo711

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

9. Oct 19, 2011

### grzz

Is the rocket moving vertically upwards?

10. Oct 19, 2011

### jojo711

Yes.

11. Oct 19, 2011

### grzz

Try to use conservation of energy.

12. Oct 19, 2011

### jojo711

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

13. Oct 19, 2011

### grzz

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

14. Oct 19, 2011

### 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

15. Oct 19, 2011

### jojo711

it's not moving, right...?

16. Oct 19, 2011

### jiboom

i dont know. have you used the suvat equations and worked out the value for the speed?

17. Oct 19, 2011

### jojo711

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

18. Oct 19, 2011

### 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 :(

19. Oct 19, 2011

### jojo711

same here! this website pretty much saved my life!