# Hitting a rocket with a projectile

• MichaelTam
In summary: You need to put { and } around the subscripts on the v, otherwise only the y becomes a...In summary, the projectile hit the rocket just when the rocket reached its maximum height. The downward gravitational acceleration is 𝑔.

I had post all of m steps and assumptions, but I don’t know where I am wrong?

Is this time I get the correct dimensions of both A and B?

MichaelTam said:
So the A must be in two dimension and B is 4 dimension, or A should be 1 dimension and B should be 3 dimension?
Sounds like you have not learned dimensional analysis. You should, it's quite easy and very useful.

I know, but I find don’t know if 3Ag is in two dimension, and## 4dB^2## is in 4 dimensions respect to time?Can I count the g as a dimension for 3Ag?And is ##4dB^2## is not in 4 dimension but in 3 dimension?

I just remember the analysis...when you tell me up about that tool.

MichaelTam said:
I know, but I find don’t know if 3Ag is in two dimension, and## 4dB^2## is in 4 dimensions respect to time?
[Ag]=LT-2LT-2=L2T-4
[dB2]=L(LT-4)2=L3T-8.

So it still incorrect?But where I am wrong...Is my assumption of ##v_{x,0}## is not correct or I make mistake on the ##v_{y,0}##?

MichaelTam said:
So it still incorrect?But where I am wrong...
The point about using dimensional analysis is that you can check each step. Use 'binary chop'. Start in the middle of your working; if it's ok there go to three quarters of the way , etc.

Ok, I am trying

My assumption in the
## v_{y,0}-(gt_f/2)= (At_f/2) - (Bt_f/12)## ( after simplify ) is not correct in dimension ?

I still can’t find out how I get wrong although I know the thing I said before is wrong, I don’t know how to fix it...

Your last line in post #36 is ##\tan(\theta)=\frac{3A(A+2g)}{4Bd}##, which is dimensionally correct and looks like the right answer to me.
How do you know it is wrong?

You mean plus or equal of the symbol?

MichaelTam said:
You mean plus or equal of the symbol?
=

Thanks!

I pass the course!,!,!

Hamiltonian
Thanks!You a lot!Very much!

Hamiltonian and etotheipi

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