Static friction and kinetic friction

[dahmsom]

I have a question on number 42. The answer says it's A. But the answer explanation says when the surface is perfectly flat the normal force equals my. The angle is defined as 0 So n must equal mg cos theta for the normal force equal mg

Why isn't the answer b? It says inclined plane??

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[dahmsom]

This is from Berkeley review physics pg 117

 

[dahmsom]

Also how do you relate incline angle to static friction . I'm having a hard time grasping the concept

 

[MrRed]

It's all trigonometry.

Friction (kinetic or static) is always the friction constant x the normal force.

Normal force is always perpendicular to the surface and equal in magnitude and opposite in direction as gravity. So figure out what component of the force is perpendicular (in this case it is the force of gravity) and whatever component that is (either sin theta or cos theta) times the friction constant, is the normal force.

 

[dahmsom]

Thanks @MrRed I was wondering how can you relate the incline angle to static friction? is it F= mgsin theta times static friction? is it that as the incline angle goes up static friction goes up... I'm having a hard time seeing proportions.

 

[jb94mg]

The reason the answer isn't B is because when you're dealing with kinetic friction, you're solving a vector problem. Friction is a contact force that's parallel to the surface, so you know that the cosine function is the only one that's going to be relevant to a kinetic-friction force problem. C and D are wrong because this isn't a static friction problem. The kinetic friction force is going to be in the direction opposite the motion of the block in a free-body diagram - that is, it's going to be parallel to the surface, and the cosine function is going to give you that force.

 

[edgerock24]

View attachment 193691

I have a question on number 42. The answer says it's A. But the answer explanation says when the surface is perfectly flat the normal force equals my. The angle is defined as 0 So n must equal mg cos theta for the normal force equal mg

Why isn't the answer b? It says inclined plane??

Quick explanation: Fk = uk * Fn
Fn for an incline plane is ALWAYS mgcos0
Therefore, Fk = uk * mgcos0.