How does object shape influence its collision?

I created a simple 3d arrow (a box deformed from one side) and imported it to unity. Then I added a rigidbody and a sphere collider to it.

When the sphere contains the arrow inside it, the arrow interacts with terrain like the sphere, which is ok.

But when the sphere is inside the mesh, it just falls to the ground without any rotation afterwards.

1)I have no idea what does collider mean, as the unity script reference defines it as “A base class of all colliders”. I thought it is some kind of hitbox, but my object’s behavior in the second case, shows that I was wrong.

2)Besides, without adding a rigidbody the physics does nothing. So, it should somehow interact with the collider. Is it explained somewhere?

3)I need to specify a sub-object to my arrow which collision can easily be calculated by the engine. How can I do it?

Add physics material to your sphere collider and check behaviour of your collider be changing the values of physics material. Add rigidbody to sphere and set drag , angular drag etc and keep checking then you will absolutely find how the shape influences the collision.

I forgot that the engine uses quite approximate physics models. The sphere falling from the height got no angular velocity, so it looked like a fall of a box.

Rigidbody reacts to physics

Colliders are for registering another collider has collided with the primary collider

A lot of times you will need a rigidbody and collider on the same object.