Contact force=?

[farmin]

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[BOX 2]
[BIGGER BOX 1] --------->acceleration and applied force

What would have to occur in order for the top box to begin to slide?
A. The contact force would have to be larger than mu(m2)g
B. The contact force would have to be larger than mu(m1)g
C. The contact force would have to be larger than the applied force, F
D. The top box would not begin to slide under any conditions.

The answer is A. I'm confused about how contact force is different from mu(m2)g. Aren't they all the same thing? What is contact force anyway? Isn't it just friction caused by BOX 1?

 

[monkeyvokes]

mu m2 g is the force of friction. if the force of friction isn't equal to the force from box 1 being pulled, then box 2 will slide off. They must just be calling force from box 1 being pulled the "contact force" i guess

 

[benjaminl1nus]

According to that law by Newton, objects at rest will stay at rest unless a net force acts upon them. in order for there to be a net force on box 2, there must be a force that exceeds the force that is already acting upon it (we know this to be friction since there is a net acceleration on the system and yet the top box remains still). if there were no friction between the two boxes then box 1 would fall off box 2 as it accelerates to the right.

anyways, in order for there to be a net force on box 2, you must supply a force (which in this case they call a contact force) that exceeds the force of friction, which is mμ(static)g. Make sense?