# How to get the force, when two objects collide at a point?

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Hi, I was wondering how to get the force vector when two objects collide when both the objects are spinning(angular velocity) as well as moving towards each other(linear velocity)? THanks

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Figure out how much the angular velocity contributes at the point of impact. The impact normal will be the same whether the objects are spinning or not (unless you're factoring in friction to the impact normal, which is difficult). The force vector will be along the same line as the impact normal. The magnitude of the force vector is different depending on the properties of your objects.

(You should probably google search for "rigid body dynamics" for exact equations and implementations)

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As you read stuff, make sure you understand the difference between force and impulse. The way collisions are represented in rigid body solvers, impulses should be used to resolve them, not forces (since a rigid body is, by definition, not deformable, so the collision takes place over zero time duration).

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hmm what if you got say two cubes:

and one is just sitting there in space, and the otherone is spinning. and the spinning one hits the still one on the edge, wouldnt it send the other one spinning or moving depending?

Thanks

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It's difficult to say really. At the point of collision, two equal and opposite forces are applied to the objects along the direction of the collision normal. The magnitude of the forces can be calculated from the penetration depth of the collision and the 'hardness' of the material.
Linear part of the force = fN
Angular part (torque) = r X fN

where:
r is the vector going from the center of the object to the collision point
N is the collision normal
f is the magnitude (I think this parameter requires a bit of guesswork/hacking to get right). Maybe something like f = penetrationDepth * hardness ?

Having said that, I could be completely wrong about everything.

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Quote:
 Original post by Anonymous PosterIt's difficult to say really. At the point of collision, two equal and opposite forces are applied to the objects along the direction of the collision normal. The magnitude of the forces can be calculated from the penetration depth of the collision and the 'hardness' of the material.

Whilst easy to implement, this will only sort-of work, will be very "unphysical", and the behaviour will be very springy. I would suggest following up what Nypyren suggests (except substitute impulse where he wrote force!).

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