But suppose we have

only interest in the motion of this direction,

then we can regard this system that has

some rolling element that

is connected by the some spring which I call k_1.

That represent the stiffness of tyre in this direction.

Of course tyre has a some mass.

So I have a mass over here.

Everybody knows that the tyre is

connected to the car with

a suspension system that we can

say the suspension system's stiffness is,

can be represented by k_2.

Then we have certain another mass which will be sort of

effective mass of car in moving that direction.

So, I have another mass over here,

and the passenger riding over this mass,

so we have interests about the motion of this mass.