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now a perpendicular force is exert on the lever, from its left end.

the lever will obviously rotate relatively to the fixed axis.

(the force will rotate with it, and will always be perpendicular to the lever. so the lever has a fixed rotational acceleration)

we know the axis of rotation will be the right end, because we have experience in life.

but if we DIDN'T know that, and only know what forces are exerted on the lever, how could we know where will be the axis of rotation?

another example is,

a body is on a frictionless floor. two forces are exerted on it, where will be the axis of rotation?

picture:

http://img573.imageshack.us/i/pamd.jpg/