1. The problem statement, all variables and given/known data Alright, I understand that friction needs to be parallel to the surface, but I'm confused on why. So if a string is attached to a block that connects to hanging masses in a lab, and you're pulling the block to measure the friction force, why does it have to be parallel to the surface? and would the frictional force change if the string were not parallel? 2. Relevant equations no equations. 3. The attempt at a solution I think that it has to be parallel so the object does not slide or move? and the frictional force would change it the string were not parallel i just don't know why... maybe it would cause the force needed to pull the block to increase? i'm confused on this concept. help?