 # Introduction

In this article Galilean transformation and inverse Galilean theory , we are going to discuss about the basic idea of Galilean transformation, various transformations , inverse Galilean transformation , also about the different laws in inverse Galilean theory.

## Galilean transformation and inverse Galilean theory

### Galilean transformation

If we know the position of a particle with respect to a stationary frame of reference, then a transformation relation through which we can find the position of same with respect to another Inertial frame of reference is called Galilean transformation.

1. #### Transformation of position- case 1

When both frame of reference coincide initially at time =0 , now S’ is moving with constant velocity v then we have P is the point of observation.

r= position vector of P with respect to S frame.

r’ = position vector of P with respect to S’ frame

From diagram we have,

OO’=Vt

r’+ Vt= r

r’ = r-Vt           (1)

If Vx,Vy,Vz are the components of velocity then we can write,

V= Vxiˆ+Vyjˆ+Vzkˆ          (2)

Similarly, the position vector in component form,

r= xiˆ+yjˆ+zkˆ                   (3)

r’ = x’iˆ+y’jˆ+z’kˆ              (4)

Now, put the value from (2),(3) and (4) in equation (1)-

x’iˆ+y’jˆ+z’kˆ=(xiˆ+yjˆ+zkˆ)- Vxiˆ+Vyjˆ+Vzkˆ

Now,compare the cofficient of i,j and k

x’= x-Vxt            (5)

y’= y-Vyt            (6)

z’= z-Vzt            (7)

Equation (5),(6)&(7) are called  transformation Galilean equations when S’ is moving in any direction.

If S’ is moving only in x-direction then equation will be-

x’= x-Vt

y’= y

And, z’=z

(Where Vx,Vy and Vz are zeo)

Case 2-

When both frame of reference are not coincide initially. Now S’ is moving with constant velocity v,then we have From diagram we have

r’= (r-Vt)-r₀

##### Transformation of displacement between two points- OP= r₁

OQ=r₂

O’P= r’₁

O’Q= r’₂

Where P,Q are point of observation

d=distance between points with respect to S frame of reference

d’= distance between two points with respect to S’ frame of reference

Now, from diagram we have

PQ = d= r₂−r₁

d = r₂−r₁              (1)(with respect to S frame of reference)

From Galilean transformation we know,

r’ = r-Vt (general equation)

For first point

r₁’=r₁-Vt                (2)

For second point

r₂’=r₂-Vt              (3)

d = r’₂−r’₁           (4) ( with respect to S’ frame of reference)

Now, from equation (2),(3)& (4)

d’ = r’₂-r’₁

d’ = (r₂-Vt  )-(r₁-Vt)

d’=(r₂-Vt )-r₁+Vt

d’= r₂−r₁

d’ = d

Distance between two points remains invariant in Galilean transformation.

###### Transformation of velocity

Let u is the velocity of particle in S frame of reference . And u’ is velocity of particle in S’ frame of reference.

Hence,

u = dr/dt and u’ = dr’/dt               (1)

Where r and r’ are position vector of S and S’ frame of reference.

We know that time coordinates can be given by-

t= t’

∴ dt= dt’                                          (2)

From Galilean transformation we have,

r’ = r-Vt                                           (3)

Differentiate equation (3) with respect to time t,

dr’/dt = dr/dt – d/dt(Vt)

dr’/dt = dr/dt -V

u’= u-V                                        (4)

Where u’ = velocity of particle in S’ frame of reference

u= velocity of particle in S frame of reference

And V = velocity of S’ frame of reference

###### Transformation of acceleration-

Let ‘a’ is the acceleration of particle in S frame of reference and ‘a” is acceleration of particle in S’ frame of reference.

From  Galilean transformation of velocity we get-

u’ = u-V

Differentiate with respect to time t,

du’/dt = du/dt-dV/dt

a’=a

(Because dV/dt =0 , V= constant)

Where a’= du’/dt

And a= du/dt

If there is no force the acceleration in both  frame of reference will be zero. That means law of inertia is followed or these are Inertial frame of reference.

###### Newton’s relativity or Galilean invariance law / classical relativity

Results are same for an experiment in both frame of reference. One is stationary and other one is moving with constant velocity . It is impossible to determine an absolute velocity or motion in inertial frame of reference.

We can calculate only relative velocity or motion between two frame of reference, this is called Newton’s relativity or Galilean invariance law.

(It is applicable only when V<<C when V ≈ C then this theory fails)

1)  Invariance of the first the law of Newton-
We that acceleration a= a’, under Galilean transformation.if there is no force acting then acceleration becomes zero, or constant. This means acceleration in both frame of reference is constant or Zero. So that law inertia is valid.
Law of inertia-
If a body is in rest will remain in rest or if a body is in moving, it will remain in the motion untill or unless an external force is applied on the body.
a=a’
2) Newton’s second law-
From Galilean transformation of force we know,
F=F’
Or dP/dt = dP’/dt
This shows that rate of change of momentum is proportional to the applied force and in the direction of force ,which wtoNewton’s second law.
3)Newton’s third law-
We have already proved that force remains invariant underiGalilean transformation that means in each frame of reference action and reaction forces are same or remains invariant. This is called third law of Newton’s.
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