What is force of static and kinetic friction? [closed]

While I am asking more than one question in this thread they are all small concept tests that you can answer with a yes or no, and is all related to me understanding kinetic and static friction.

What I know:

Force of friction = Mu*N.

Static friction = The friction that keeps the object from moving.

Kinetic friction = The friction that is pushing against the motion of the object.

What I don't think I understand/or what I think is the problem:

I don't think I know what Mu(s), and Mu(k) really are, and I might be mixing them (or know if they are even one).

What I don't understand:

According to a physics teacher the force of static friction /= Mu(s)*N except sometimes.

How is that?

Qustion number 2 in the picture. http://gyazo.com/ad3f358af9f36c1ddcb2e8b4d93188c2

If an object is standing still it's static so how can the answer to the question be false?

The same box as in question 1. Question 2. http://gyazo.com/b5869084b618e96e6a1fe491274295f4 The answer is yes.

I think this one is yes because for the block to move the tension in the string needs to be bigger than the maximum static friction.

The same box as in question 1. Question 3. http://gyazo.com/52af207d0c0b55453cc200b875d3e179

Question 4. http://gyazo.com/52af207d0c0b55453cc200b875d3e179 It must be moving right? But not accelerate.

closed as too broad by David Z♦Jul 27 '15 at 2:37

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

• why didnt you use the trusted image service imgur provides that is inlined, I dont know what gyazo is. Re "How is that": the thing is, its an heuristic that works most of the time, there is no fundamental reason why it should be strictly proportional, but it makes sense that it is linear at least for small driving forces and experimentally it matches – Bort Jul 26 '15 at 19:58
• You can search Gyazo up. It is completely safe and one of the most popular screen-shot capturing programs out there. – David Lund Jul 26 '15 at 20:03