PHYSICS: So why don’t things that are moving just keep moving? Here’s the intro lecture on friction.
Remember a few key things. Net forces produce acceleration. You can find Ff a couple of different ways – it’ll be part of a net force equation and can be found from the coefficient of friction equation. If and ONLY if your object is moving at a constant velocity, Ff will equal Fa. Fn is only sometimes equal to Fw. If your object is pulled at any angle (or pushed), Fn will be a part of Fnet in the vertical direction.