# How do you calculate drag to make a rigidbody stop at a given position?

Discussion in 'Physics' started by jlink, Sep 5, 2019.

Joined:
Jan 8, 2017
Posts:
6
I have a projectile that uses physics that I want to stop at a given position. I can apply a force in the direction of the position to make it cross the position, but how can I calculate and set drag so that the object stops at the target position?

As a bonus question, how can I calculate the drag and velocity required to make the object stop at a given position AND given time?

Joined:
Jan 8, 2017
Posts:
6
* bump *

3. ### xorpheous

Joined:
Mar 28, 2018
Posts:
19
The equation you're trying to solve is (d^2/dt^2)r = -g - (C_d / m)* |dr/dt|^2 * dr/dt

You can't solve this explicitly for r(t) = stuff. The best you can do is model it numerically. You CAN solve for the non-drag solution in closed form using the constant acceleration equation, r(t) = r_i + v_i * t + (1/2) a t^2. If you're going over flat terrain, then your total range is |r| = (v^2 / g) * sin(2 * angle). Again, that's without drag.