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Force = Mass x Acceleration
or,
f = ma
where f stands for Force, m for Mass, and a for Acceleration.
or,
dv
f = m --
dt
which can be written as
dv f
-- = /
dt m
because acceleration is the rate of change of velocity, v. And
what we have there is a differential equation. The left-hand side
isn't exactly a fraction, dv divided by dt; that's just
a way of showing that the equation tells you how fast v changes
over time, rather than what the fixed value of v is.
The way it works, suppose m=1 and f=2. Then f/m=2, so v increases by 2 every second. So you know that if v starts at 0, then after 4 seconds v=8.
This is a pretty easy differential equation, so easy in fact, that if (as we suppose here) f and m have fixed values, we can solve it in our heads to see what v is at any particular time, even time 1,000,000. The differential equations that mathematical biologists work with are more complicated, so we can only solve them approximately, on computers.