Mathematical Formula For Newton’s First Law of Motion
It is very strange to note that, we have
mathematical formula for Newton’s second law of motion F = ma, but no formula
for Newton’s first law.
Simple Derivation of Newton’s Ist law From 2nd Law
We know the relation between force and linear
momentum
F
= (Pf -
Pi)/tf -ti
F
= DP/Dt
(Rate of change of linear momentum)
As an analogy
P
= m(xf -
xi)/tf -ti
P
= mDx/Dt
P = mv {v = Dx/Dt}
Momentum of a body is equal to the rate of change of
change of position of a body of mass m which is nothing but Newton’s first law
If the position is not changing then its momentum is
zero means the body is at rest P = 0
Usual Derivation of of Newton’s First Law
In the absence of an unbalanced force, linear
momentum of a body is directly proportional to the its velocity and inversely
proportional to its mass
(1) vµ
P
(2) vµ
1/m
Combining (1) and (2), we have
vµ
P/m
v
= k P/m
when k =1 then
v
= P/m
P = mv
Linear momentum is equal to the rate of change of
position of a particle of mass m
Newton’s 2nd Law of Motion from First Law
Writing momentum as the rate of change of posion of
a body of mass m
DP = mDx/Dt
Dividing both sides by Dt
DP/Dt = mDx/DtDt
F
= mDv/Dt
F = ma