Re: [Phys-l] Collision of irregular bodies

[John Denker <jsd@av8n.com>]

D.V.N.Sarma wrote:
> When two bodies of smooth surface but irregular shape collide 
> what is the line along which the action reaction pair acts?

Short answer:  Nobody knows, not without significantly more
information.  Even in the absence of friction (which may or
may not be what was intended by saying "smooth surface")
the outcome depends on details such as the distribution of
mass within the objects.

And I'm not even sure the question makes sense.

In such a collision, you need to know two things:  how
much momentum is transferred, and how much angular
momentum is transferred.  I suppose you could define a
"line of action" but I'm not sure it would tell you
what you really want to know about the collision.

==========

This has serious practical consequences.  Suppose for
instance you are chopping down a tree that stands close
to the north side of a house.  Even assuming you arrange
that the top of the tree falls northward away from the
house, the tree will have a pronounced tendency to
pivot around its center of mass, which means the base
of the tree could very easily end up considerably south
of the stump, destroying the house.

Of course proper tree-felling technique will prevent
this from happening, but you reeeally want to use the
proper technique.  Don't try it unless you're sure you
know how.