Newton's Second Law
In the presence of a NET FORCE, an object experiences an ACCELERATION
directly proportional to the NET FORCE
inversely proportional to the MASS of the object.
Remember, though,
 F is the NET force
 m is the mass which that net force acts on.
We often turn this around and write it as
F is the NET force acting on an object
and
m is the mass of the object which the force F acts upon.
What are the UNITS of force in
F = m a A force of ONE unit
will give an object of 1.0 kg mass
an acceleration of 1.0 m/s/s ;
this force is known as
ONE NEWTON (1.0 N) .
1 N = ( 1 kg ) ( 1 m/s/s )
F = m a Force will be measured in newtons
A force of 1 N will give
a mass of 1 kg
an accelertion of 1 m/s/s.
12 N = ( 3 kg ) ( 4 m/s/s ) A force of 12 N could give a mass of 3 kg
an accelertion of 4 m/s/s
12 N = ( 2 kg ) ( 6 m/s/s ) A force of 12 N could give a mass of 2 kg
an accelertion of 6 m/s/s .
We have already seen that all objects fall with the same acceleration, 9.8 m/s/s (which we approximate as nearly 10 m/s/s).
We call this free fall.
When such objects fall, the only force acting on them is their weight, the force of gravity.
The only force on a body in freefall is the force of gravity. We call this its weight.
Since it accelerates at 9.8 m/s/s,
that weight must be
w = (mass ) x (9.8 m/s/s),
w = m g
The weight of an object is the force of gravity on that object.
Weight, since it is a force, will be measured in units of newtons (N).
Mass will be measured in kilograms (kg).
If an object is in equilibrium  at rest  then, the net force on the object must be zero.
That is, the sum of all the forces on an object is zero when the object is in equilibrium.
