CM [009] Newton's First Law

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NEWTON’S 1ST LAWS OF MOTIONCM [009]


[and] a particle in uniform rectilinear motion will continue to move on forever at constant speed in the same manner.

Newton’s First Law in 3 Parts

A particle in rest will remain forever at rest

It will change its state of motion only and only when it is compelled to do so by forces impressed on it.

Law No. 1 can be more conveniently studied in three parts I , II, & III.


Newton’s First Law : Part 1

A particle will remain at rest and stays like that forever. Why is it not moving? Because it is in the situation when there no momentum. So a particle at rest is not a special case of motion. It is a motion with 0 momentum.


𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉 = 0

So momentum:𝑝𝑝 = 𝑚𝑚𝑚𝑚 = 0

So I am not moving




This is the old description of impetus and conservation familiar to John Philoponus, Jean Buridan, Descartes and Galileo Galilee. This will be studied in the coming sections.



It will change its state of motion only and only when it is compelled to do so by forces impressed on it.

This part deals with force and its effect on the object’s state of motion. So it is also to be dealt with in a later section on force and acceleration.


Law 1 Part 3

Law 2 Law 3

Sequence of Analysing Newton’s Law

We start from here!




This is not an independent case. It is only a special case when momentum is equal to zero. So it can be incorporated into Part 2 as a subsidiary, saying:When a particle has no momentum, it will not move, thus remains at rest.

𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉 = 0So momentum:

𝑚𝑚𝑚𝑚 = 0So the object is

not moving




This phenomenon had been demonstrated by Galileo before in his ramp experiment. When there is no friction and air resistance, the ball will move on forever.


Comparison

When we compare this modified law with Newton’s original one, it can be seen that the latter is a description of a physical phenomenon. The new law spells out the more basic elements that is affecting motion. First we have the motion, then we have the event described by Newton’s First Law.

Momentum

A particle in rest will remain forever at rest, and a particle in uniform rectilinear motion will continue to move on forever at constant speed in the same manner. It will change its state of motion only and only when it is compelled to do so by forces impressed on it.

An object is moved by its momentum.

Newton’s Modified First Law Newton’s Old Descriptive First Law


Newton’s First Law of Motion - Modified

By eliminating Part 3 and incorporating Part 1 into Part 2, we have a more basic law of motion – Law No. 1:

An object is moved by its momentum.

The other situations can be written as corollaries:

i. Momentum never fades away and so is conserved.

ii. When momentum is zero, the object will be rest.

iii. An object travels at the direction of its momentum.iv. Since momentum is conserved, constant motion is for ever.


Object Carried by Momentum

Instead of saying an object is motivated by its momentum, it is more correctly to say that an object is carried by its momentum: like a man carried by a swan or a saint carried by angels.


. . . and the deeper foundation of the First Law of motion is momentum, not force.


NEWTON’S 2ND LAW OF MOTIONTo be continued on: [010]

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CM [009] Newton's First Law