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.




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).