Newton's Laws redux.

1. Newton's First Law (Inertia)

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

That 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 the force is unbalanced - greater than any resisting forces), the object will accelerate.

Symbolically:

F = m a

The Force (F) on a mass (m) produces acceleration (a), predicted by the above equation. In detail:

Greater F means greater a
If the Force is kept constant, but the mass is increased, the acceleration will be smaller:

a = F/m

That's an inverse relationship.


There is a new unit for Force - since Force = mass times acceleration, the units are:

kg m/s^2

We give this a new name, the newton (N). It's about 0.22 lb.


Weight:

There is a special force, the force due to gravity. It's called weight (W).


W = mass x acceleration (due to gravity)

or

W = m g

This is worth noting - there is a BIG difference between mass (m) and weight (W). Mass is the amount of stuff there is and weight is the extent to which it is pulled to the Earth (or wherever).

Since g on the Moon is around 1/6 that of Earth, your weight on the Moon would be around 1/6 of your Earth weight.


3. Newton's 3rd 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.

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 experience the same force:

m A = M a

That's a little tricky to convey in letters but, the larger object (M) will experience the smaller acceleration (a) and the smaller object (m) will have a larger acceleration (A).