Collision between a "spring" and a particle I am currently attempting to create a simple 2D physics engine, and I just need to know how to solve a certain collision. 
I have particles, and between any two particles, springs can be connected. These springs can have variable tension and length. Now let's say I have another particle that collides with this spring. I would like to resolve this collision as if the spring couldn't bend when the particle hits it, but rather it just behaves as a stick would in the real world. I am storing the velocities and positions of all of the particles, not the angular momentum, so how should I update the velocities of all the particles?
 A: Find the center of mass (CM) of the two connected particles. Then, determine the distance (r) from the center of mass of the part of the spring where the third particle hit it. 
Then, if you could allow a collision time and force while in contact, and assume the collision to be frictionless (this is might be hard for point particles and thin spring, cause deceleration time means it might just have enough time to cross the other side), you could use T = Fr(r is the distance of CM to the hit part), assume the direction of force is perpendicular to the spring (frictionless) and F = Ma, then T = IA = Fr.
 For the translational effect of the two particles:
Use F = Ma to find the acceleration of center of mass, a = F/M, where M is the sum of the mass of two springs, then apply the acceleration to the two particles (acceleration is same direction of the force which is perpendicular to spring).
For the rotational effect: Fr = IA (A is angular acceleration, I is moment of inertia :I =  m1(r1)^2 + m2(r2)^2, r1 and r2 are distances of the 2 particles from center of mass), so A = Fr/I. Then (let Q1, Q2 be tangential acceleration of particle 1& 2), Q1 = r1*A, Q2 = r2*A. We can then apply the tangential acceleration to the two particles. if the part of the spring hit is between CM and particle 1, Q1 would be the same direction as the force, and Q2 would be the opposite direction. if the part of the spring hit is between CM and particle 2, Q2 would be the same direction as the force, and Q1 would be the opposite direction.
For the third particle that hit it: Just  F = ma. but the direction of this force is equal but opposite in direction to the above force (law of equal and opposite reaction)
As for the force to be used, you could use a force that is dependent on the (nearest) distance of the third particle to the spring, and activates when a certain distance is reached (say 0.5 units). Then perhaps use increasing force when they get nearer and nearer (like a bouncing ball), say for example, Lennard Jones potential, where F = -dU/dx.
