Hello I took a general physics course 2 years ago, and now I am trying to refresh a bit on my knowledge of physics so I started solving discussion questions+problems in University physics by young My problem is I can't verify my answers nor can I look for the correct answer if I don't know it, since it isn't included in solution manual, I mean the normal exercises are, but the more discussion questions aren't 1. The problem statement, all variables and given/known data Like for instance this question Q5.18) You often hear people say that "friction always opposes motion" Give at least one example where a) static friction causes motion, b) kinetic friction causes motion 2. Relevant equations 3. The attempt at a solution Well for static friction I would think the the friction between our feet and the ground causes walking i guess? I mean when you push your feet on the ground the reaction force would be considered a static friction force, but I don't know about kinetic friction. I also made this not for this question specifically it's just that the book no matter how many times I re read it, I don't find answers to these question, so how exactly am I supposed to answer them? ==================================================================== Also there is this question that says can a body be in equilibrium when only one force act on it I think no but I can't be 100% sure cause let say you put an metal in an air free container in middle of space and position a positively charged particle exactly above it the way I think of it since electrons would be going in orbits wont the object affect the individual protons in the atom I know the atom is by itself neutral but wouldn't there be a coloumb repulsive force between that charged object and the protons in the metal even though it wont move?