Newton, Laws II and III

Lex. II. Mutationem motus proportionalem esse vi motrici impressae, & fieri secundum lineam rectam qua vis illa imprimitur.

The alteration of motion is ever proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressed.

If any force generates a motion, a double force will generate double the motion, a triple force triple the motion, whether that force be impressed altogether and at once, or gradually and successively. And this motion (being always directed the same way with the generating force), if the body moved before, is added to or subtracted from the former motion, according as they directly conspire with or are directly contrary to each other; or obliquely joined, when they are oblique, so as to produce a new motion compounded from the determination of both.

Lex. III. Actioni contrariam semper & aequalem esse reactionent: sive corporum duorum actiones in se mutuo semper esse aequales & in partes contrarias dirigi.

To every action there is always opposed an equal reaction; or the mutual actions of two bodies upon each other are always equal, and directed to contrary parts.

Whatever draws or presses another is as much drawn or pressed by that other. If you press a stone with your finger, the finger is also pressed by the stone. If a horse draws a stone tied to a rope, the horse (if I  may so say) will be equally drawn back towards the stone: for the distended rope, by the same endeavour to relax or unbend itself, will draw the horse as much towards the stone as it does the stone towards the horse, and will obstruct the progress of the one as much as it advances that of the other.

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And now, in more contemporary language:

1.  Newton's First Law (inertia)

An object will keep doing what it is doing, unless there is reason for it to do otherwise.

The means, it will stay at rest OR it will keep moving at a constant velocity, unless acted on by an unbalanced force.

2.  Newton's Second Law

An unbalanced force (F) causes an object to accelerate (a).

That means, if you apply a force to an object, and that force is unbalanced (greater than any resisting force), the object will accelerate.

Symbolically:

F = m a

That's a linear relationship.

Greater F means greater a.  However, if the force is constant, but the mass in increased, the resulting acceleration will be less:

a = F / m

That's an inverse relationship.

We have a NEW unit for force.  Since force = mass x acceleration, the units are:

kg m / s^2

which we define as a newton (N).  It's about 0.22 lb.

There is a special type of force that is important to mention now - the force due purely to gravity.  It is called Weight.  Since F = m a, and a is the acceleration due to gravity (or g):

W = m g

Note that this implies that:  weight can change, depending on the value of the gravitational acceleration.  That is, being near the surface of the Earth (where g is approximately 9.8 m/s/s) will give you a particular weight value, the one you are most used to.  However, at higher altitudes, your weight will be slightly less.  And on the Moon, where g is 1/6 that of the Earth's surface, your weight will be 1/6 that of Earth.  For example, if you weight 180 pounds on Earth, you'll weight 30 pounds on the Moon!

3.  Newton's Third Law

To every action, there is opposed an equal reaction.  Forces always exist in pairs.  Examples:

You move forward by pushing backward on the Earth - the Earth pushes YOU forward.  Strange, isn't it?

A rocket engine pushes hot gases out of one end - the gases push the rocket forward.

If you fire a rifle or pistol, the firearm "kicks" back on you.

Since the two objects (m and M, let's say) experience the same force:

m A = M a

That's a little trick to convey in letters but, the larger object (M) will experience the smaller acceleration (a), while the smaller object (M) experiences the larger acceleration (A).