Re: apples and oranges

["Brian McInnes" <bmcinnes@pnc.com.au>]

Ed, 
For a given object, the acceleration  divided by the net force always gives
the same number (the mass of the object).  That relation is called a
proportion.  It is said that the acceleration and the net force are
proportional to each other.  By division we can find out how much
acceleration goes with one unit of net force.
Alternately we can find out how acceleration we have with a different value
of the net force. Suppose unit force (1N) led to an acceleration of 2
m/s*-2.  A force of 60 N  means we have 60 lots of unit force and to find
the resultant acceleration we have to add up 2 m/s*-2 60 times - once for
each 1 N force. The fast way to do this is to multiply 60 x 2 m/s*-2, which
is 120 m/s*-2.  We use multiplication because we want to add the same
quantity (the acceleration produced by unit force) repeatedly.

The above argument is adapted from  a treatment of density, mass and volume
in McDermott's Physics by Inquiry.

Brian McInnes 
----------
..  How come we can multiply apples and
> oranges and get something reasonable?  That is, why is it
> that mass times acceleration yields something useful when
> addition of mass and acceleration does not?