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Anyway, I read a version of newton's second law as follows (I've not got it to hand so this may not be exactly right)

The change in momentum per unit time is proportional to the resultant force in that direction.

Its the word propotional that's confusing me.

If v = terminal velocity

& u = initial velocity

mass (resistance to change in velocity) is constant

then change in momentum per unit time is (mv - mu) / t

taking out a common factor of m gives m (v - u)/t

as (v - u) / t = a then we have the change in momentum per unit time = to ma. Newton's second law F = ma

Where does the 'proportional' into play? Shouldn't there be a constant of proportionality somewhere to make it an equation?

Thanks