[Physics] Multiple colliding balls

collisioncomputational physicsmomentumnewtonian-mechanicssoftware

I have written an algoritm to resolve a collision between two balls with conservation of momentum. It looks to work exactly as expected in my simulations. Here is the code:

public void Resolve()
{
  var r = 0.5; // coefficient of restitution
  var m = Ball1.Mass + Ball2.Mass;
  var ua = Ball1.Velocity.Magnitude * Math.Cos(Ball1.Velocity.Angle(Normal));
  var ub = Ball2.Velocity.Magnitude * Math.Cos(Ball2.Velocity.Angle(Normal));
  var va = (r * Ball2.Mass * (ub - ua) + Ball1.Mass * ua + Ball2.Mass * ub) / m;
  var vb = (r * Ball1.Mass * (ua - ub) + Ball1.Mass * ua + Ball2.Mass * ub) / m;
  Ball1.Velocity -= Normal * (ua - va);
  Ball2.Velocity -= Normal * (ub - vb);
}

Now I also want to make this work for more than two balls colliding at the exact same moment. My initial thought was that I could just solve this equation several times for each contact between balls. When I have a situation like a "Newton's cradle" where the balls is in contact in a "serial" way it's easy to just solve each collision on at a time and without moving the balls just go ahead and calculate the next and the next etc until reaching the last ball which will end up with all energy not lost along the way. This works fine also, albeit not very fast.

But what about for instance a case where three balls are in contact in a V-shape and the bottom ball is the one with a velocity (up in this case)? I mean how now do I distribute the energy between the other two balls? The equation I have only works for two balls and now I don't see how I can calculate this iteratively anymore. Can you help guide me in how I should think with this? Can I modify my Resolve-function to work with any number of balls? I mean with a set of $N$ balls with connecting surfaces and initial velocity vectors, what will the outcome velocities be? Is there a well known general solution for this?

Note:

I have tried out the method currently marked as the answer but I have unfortunately got a bit weird results in some setups. Be warned. I will probably revert back to resolving one collision at a time again. I will update this info when I have tried this more.

Best Answer

You need to setup a system of $K$ equations with $K$ impulse unknowns if derived from the $N$ balls whereas only $K$ of them are colliding at some time.

Here is how to do it:

  1. Create a (block) vector with all velocity vectors in sequence. The size is $2N\times1$ $$ \mathbf{v} = \begin{pmatrix} \vec{v}_1 \\ \vec{v}_2 \\ \vdots \\ \vec{v}_N \end{pmatrix} = \begin{pmatrix} \dot{x}_1 \\ \dot{y}_1 \\ \dot{x}_2 \\ \dot{y}_2 \\ \vdots \\ \dot{x}_N \\ \dot{y}_N \end{pmatrix} $$
  2. Create a (block) matrix with all the masses, but also of $2N$ rows $$\mathbf{M} = \begin{bmatrix} m_1 & 0 & 0 & 0 & \cdots & 0 & 0 \\ 0 & m_1 & 0 & 0 & & 0 & 0 \\ 0 & 0 & m_2 & 0 & & 0 & 0 \\ 0 & 0 & 0 & m_2 & & 0 & 0 \\ & & & & \ddots & & \\ 0 & 0 & 0 & 0 & & m_N & 0 \\0 & 0 & 0 & 0 & & 0 & m_N \end{bmatrix}$$ such that $\mathbf{M} \mathbf{v}$ would be all the momentum componnets.

  3. You are looking for the change in velocity $\Delta \mathbf{v}$ due to all the impulses from the collisions. If you knew the sum of all impulses $\boldsymbol{\ell}$ (again in $2N\times1$ vector) then you would write the change in velocity as $$\Delta \mathbf{v} = \mathbf{M}^{-1} \boldsymbol{\ell}$$

  4. Create a $2N \times K$ contact pair block matrix $\mathbf{Z}$. Each column represents a contact between two bodies ($i \rightarrow j$) by containing the contact normal vector $\hat{n}_{ij}$ in the $j$-th block position, and the negative normal vector $-\hat{n}_{ij}$ in the $i$-th block position. For example: $$\mathbf{Z} = \begin{bmatrix} -\hat{n}_{13} & 0 & -\hat{n}_{14} \\ 0 & -\hat{n}_{24} & 0 \\ \hat{n}_{13} & 0 & 0 \\ 0 & \hat{n}_{24} & \hat{n}_{14} \end{bmatrix}$$ represents the contact pair matrix when bodies ($1 \rightarrow 3$, $2 \rightarrow 3$ and $1 \rightarrow 4$) are colliding. The arrow indicates the direction at which the normal vector points, and hence the direction where the positive impulse is applied. Remember that each contact normal vector has two components $\hat{n} = ( \cos \psi, \sin \psi )$.

  5. Construct a $K \times 1$ vector of unknown impulse magnitudes for each contact pair. $$ \mathbf{J} = \begin{pmatrix} J_{13} \\ J_{24} \\ J_{14} \end{pmatrix}$$ Now the sum of all impulses are computed by $$\boldsymbol{\ell} = \mathbf{Z} \,\mathbf{J}$$

  6. Calculate the relative speed of each contact pair ($K \times 1$ vector) by projecting the speeds along the contact normal $$ \mathbf{u}_{imp} = \mathbf{Z}^\top \mathbf{v}$$

  7. Establish the law of collision in terms of the relative speeds before and after the collision. $$ \mathbf{Z}^\top \left( \mathbf{v} + \Delta \mathbf{v} \right) = -\epsilon \mathbf{Z}^\top \mathbf{v}$$ where $\epsilon$ is the coefficient of restitution. Move the knowns to the right hand side and find the change in relative speeds by $$ \mathbf{Z}^\top \Delta \mathbf{v} = -(1+\epsilon) \mathbf{Z}^\top \mathbf{v} = -(1+\epsilon) \mathbf{u}_{imp}$$

  8. Put 3. 5. and 7. together to make $$ \mathbf{Z}^\top \left(\mathbf{M}^{-1} \left( \mathbf{Z} \,\mathbf{J} \right) \right) = -(1+\epsilon) \mathbf{u}_{imp} $$ and solve for the unknown impulses $$ \mathbf{J} = -\left(\mathbf{Z}^\top \mathbf{M}^{-1} \mathbf{Z} \right)^{-1} (1+\epsilon) \mathbf{u}_{imp} $$ Note that $\mathbf{Z}^\top \mathbf{M}^{-1} \mathbf{Z}$ is a $K \times K$ matrix that needs inversion.

  9. When all the impulses are calculated, each body velocity vector changes by $$ \Delta \mathbf{v} = \mathbf{M}^{-1} \mathbf{Z} \,\mathbf{J} $$

Example

Imagine three spheres impacting at the same time:

pic

Use this MATLAB script to find their final velocties:

%% Example triple impact (with 3D vectors)
% Three spheres impact at the same time. See Figure for their
% masses, positions and velocities before impact

% define unit vectors
o_ = [0;0;0];
i_ = [1;0;0];
j_ = [0;1;0];
k_ = [0;0;1];

R = 20;
r_1 = R*i_ 
r_2 = -R*i_
r_3 = R*tand(60)*j_

% Three possible contacts: 1*2, 2*3, 1*3
n_12 = (r_2-r_1)/norm(r_2-r_1)
n_23 = (r_3-r_2)/norm(r_3-r_2)
n_13 = (r_3-r_1)/norm(r_3-r_1)

% velocity vectors before impact
v_1 = o_;
v_2 = -1.0*n_12
v_3 = -2.0*n_13

v = [v_1;v_2;v_3]

% mass matrix
M = blkdiag(5.0*eye(3),4.0*eye(3),3.0*eye(3))

% contact pair matrix
Z = [-n_12, o_, -n_13; n_12, -n_23, o_; o_, n_23, n_13]
% Z =
% 
%     1.0000         0    0.5000
%          0         0   -0.8660
%          0         0         0
%    -1.0000   -0.5000         0
%          0   -0.8660         0
%          0         0         0
%          0    0.5000   -0.5000
%          0    0.8660    0.8660
%          0         0         0

% coefficient of restitution
eps = 0.5

% impact speeds
u_imp = Z.'*v

% impulses
J = -(1+eps)*inv(Z.'*inv(M)*Z)*u_imp
% J =
% 
%     1.7021
%     2.1702
%     4.6277

% velocity vector change
Dv = inv(M)*Z*J

% final velocity
v_final = v+Dv
% v_final =
% 
%     0.8032
%    -0.8015
%          0
%     0.3032
%    -0.4699
%          0
%     0.5904
%     0.2303
%          0


% check for error
actual = Z.'*(v+Dv);        %(relative speeds after impact) =
expected = -eps * u_imp;    % -eps*(relative speeds before impact)
err = actual-expected

% err =
% 
%   1.0e-015 *
% 
%    -0.1110
%    -0.6661
%    -0.6661

In the last part, I check the results against the law of contacts and the error is ~1e-15.

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