How to get Physics2D collision force

Unity Engine
, , ,
1

I have a 2d game where a ship collides with meteorites.
Based on how hard was the impact I will then detract ship health.

To me this sounds like a very simple thing but I was unable to find this collision info for Collider2D.

What am I doing wrong?

2

well, you know what hit you should be able to get its velocity

3

You’ll need to be specific, there’s no such thing as “how hard” in physics. All I can point out is what’s already there but you said you were unable to find it so I’m not sure how else to help.

As you’ve already seen:

but there’s a huge difference between a feather hitting you at 1 m/s and a car so you can scale this by mass.

… and per contact you get:

The normal impulse is what was required to keep them separated and could work well for you.

4

Thank you! That was exactly what I was looking for.

For posterity here is the code I ended up with:

private void OnCollisionEnter2D(Collision2D collision)
    {
        var impactForce = collision.contacts.Sum(x => x.normalImpulse + x.tangentImpulse);

        var damage = Mathf.Max(0, impactForce - _collisionResistance);
        
        Debug.Log($"Damage taken: {damage}");
    }
1 Like
5

I’d avoid using Linq but that’s your choice. Also, using the “contacts” field also produces garbage as it has to create a list.

You can use the non-allocating stuff for performance:

Reuse a List in your script. It’ll automatically have the correct capacity but after a few calls you won’t have any allocation etc.

1 Like