# Projectile trajectory : calculating gravity and height

Hello,

I'm trying to simulate a projectile movement (without any air friction). But my projectile parameters are not conventional.

I know :
1. The initial height (y0)
2. The initial velocity (v0)
3. The maximum horizontal travel distance (d)
4. The maximum height (h)

I need to find :
• the initial angle (θ)
• the gravity (g)

So basicly I'm trying to find the formulas that give θ and g according y0, v0, d, h.
And that's where I'm stuck :)

Here is an image to be a bit clearer :
[PLAIN]http://img14.imageshack.us/img14/6706/projectile.png [Broken]

Any help is welcome

Thanks !

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## Answers and Replies

rock.freak667
Homework Helper
Depending on where you are, say Earth at ground level/small heights, 'g' would be 9.81 m/s^2 i.e. a constant. You would know 'g'.

As for θ, if you consider vertical motion,

v2=u2-2gh

at max height, H, v=0 so you would be able to solve for u which would involve the angle θ.

Thanks, but actually I cannot use 'g' with 9.81 m/s^2 or any other 'constant'.

I know it is not usual but 'y0' 'v0' 'd' and 'h' are my constants. 'θ' and 'g' are my variables.

You may consider I'm simulating the projectile on an unknown planet, or if you prefer a planet where the gravity can change between two projectiles throw.

The goal behind that is quite simple actually. In my simulation I want to be able to launch a projectile at any possible distance without changing its initial velocity. So the only way to be able to do that is to modify the gravity (hard to do that on earth but not in a simulation) And since I also have another constraint, the height. I also need to find 'θ'.

Thanks

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