Back to basics: What's a force?

I don't think it's a bad idea to start with repeating the basics of the basics. Especially because we're holiday now. And so I dedicate my first post to " forces", in its wide, physical meaning.

"A force is a cause of a transformation or of a velocity change of an object." and this means a force can give an object a different form (like when you go sit on a pillow), or it can accelerate, slow down, or completely stop an object (like you can do by kicking a football). All other reactions of a force acting on an object are derived from these 4 effects.

That, you just have to keep in mind. More important in physics, is being able to measure that force and perform calculations with it. Therefore, we have unit of force: the "Newton", or simply "N". For example, if you have a bag of 1 kg on your hands, on sea level, there is a force of 9.81N pushing your hands down.

This is still very abstract, but it will become more clear overtime, after it you have performed some calculations with it. The "Newton" is also one of the only new units you have to know. Many other units are derived from it.

By the way, a force is a vector, while the size of the force (in Newton) is a scalar. So, to name a force, we use an "F" with "->" on top of it. If we talk about the size of the force, we simply use "F" (if there is more then one force, we use indexes, like F1, F2, FA(buoyancy), FG (Gravitational force), etc.)

Since a force is a vector, it also has all the characteristics of it. In maths, you might have learned, a vector is completely determined by 3 things: the point in which the vector starts, its size and its direction.
For example
In this case, the point where the force starts is "A", the size is 15N and the direction is horizontal.

These are the basics, and you should understand them. If they are not clear to you, please mail me, and I'll add more.