Can someone explain this to me.
Say if we have to forces acting
The weight and the normal force.
We can setup a equation like so (from my knowledge) - taking down as negative
-Fw = Fn
Or
Fw=-Fn
Net force = Fn + Fw
And if given a net force we can solve for the normal force acting.
Like wise with friction and the pulling motion of a subject (the horizontal component) - take left as positive
Fd=-Ff
-Fd=Ff
So can some explain why this same approach isnt set out in the Vtextbook video for circular motion?
Have they just not taken into account direction when doing the forces.
Or have i totally just screwed up with the theory?
Hi Cosec

Nice to see someone using VT

For your first equation, let's assume that we have a box resting on a table. That way we have a zero net force, and there only two forces that act on the box are the weight force,
FW and
FN.
As forces are vectors, we should put tilde, use arrows or use boldface to show that (see above). Now, if we don't assign a direction as being positive, then yes of course we can write:

Since we know that the net force is zero, we can let the left hand side be zero and move
FN to the other side:


^ which is the set of equations you got.
Now, the alternative method is to take into account the directions first, and then work with just magnitudes (which is what's done throughout the VT vids, as working with vector notation usually confuses the hell out of ppl not doing spesh).
1) Let up be positive for the box example
2) Now, 'convert' the vectors
FW equals to a force with magnitude of F
W (note no more boldface) downwards. ie:

(note how the arrow disappears)
FN equals to a force with magnitude of F
N upwards. ie:
3) So our equation for the net force becomes, using only
magnitudes since we have already taken into account the directions
 + (- F_\text{W}))

When this is rearranged, and we sub in zero for the net force, we would get:

... notice the difference in sign because we've already taken into account the direction, whereas in the first example we were still working with vectors!
As for the circular motion in VT vid, let's look at the diagram on the left:
1) Choose on which direction is positive
Take up as positive
2) Look at the forces and add the sign by looking at the direction
Normal force acts up, so it is negative: - N
Weight force acts down, so it is positive: + W
Net force acts down, so it is positive: + F
net3) Write the equation of motion
F
net = W - N
This is what it would look like if we used vectors
Apologies if this wasn't clear on the video, it's been quite a while haha. Hope it makes sense now
