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What first - Newton of course

Thinking about what I should start with?? Well of course, the root of mechanics

  Newtons 3 laws of force and motion.

1) A body remains at a constant velocity unless acted upon by an external force

Remember that velocity is displacement (movement) in a direction and that is the direction of the force that was initially applied to start the movement

When a body has velocity and mass it has Momentum 
Momentum keeps a body going in a given direction until acted upon by an outside force.

NB: Assuming a frictionless vacuum, because friction and air resistance are forces

2) Force equals mass x acceleration or f=ma ---- which also means f minus ma = 0

Which is what law 3 says

3) Every action (applied force) has and equal and oposite reacton (resisting force). So the sum of the forces is zero

How then can movement occur if all the forces are equal and opposite?

READ ON!

The net balance of forces in a system, whether in motion or stationary, always sums up to zero. Always!!

Why? Because of INERTIA!!

Inertia is a known as a ficticious or virtual force and is only apparent in the presence of acceleration. Inertia is the force that resists motion in the opposite direction of the acceleration. To have motion, there must also be an initial applied force and therefore an acceleration. Once started the body can continue in motion, without force or acceleration, due to its momentum.

So what's the difference between inertia and momentum, they both seem very similar?

Inertia requires acceleration and therefore an applied force, momentum requires velocity but no acceleration and both require mass. Momentum keeps us going, inertia want us to stay where we are.

Acceleration doesn't necessarily imply movement though.

Gravity is a fine example of this - A mass at rest on the earth experiences acceleration due to gravity, which is why it has weight (the force x acceleration of the mass acting on the ground) but the body has has no movement.

Lets expand on those notions  - Go here Newtons Laws

More in depth? - check out Newtons Laws here

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