ek physics 229 vs 230

[inaccensa]

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3 blocks with same shape and made from the same material with masses 1,2,3kg respectively are placed on a board. The board is slowly lifted at one end.

-->q1-which block will start sliding first?
here the explanation completely ignores the kinetic frictional force
fdown=fup
mg*sintheta=mustatic*mg*costheta
sinthetha=mucostheta
If the block starts sliding, it is equal to the max static frictional force,is this premise correct? We dont consider the kinetic force when the block begins to slide
-->which block will slide fastest?
mg*sintheta-mustatic*mgcostheta=ma
since acceleration is independent, they all slide at the same time. but wouldnt we consider inertia at all in this case?

 

[cham90]

3 blocks with same shape and made from the same material with masses 1,2,3kg respectively are placed on a board. The board is slowly lifted at one end.

-->q1-which block will start sliding first?
here the explanation completely ignores the kinetic frictional force
fdown=fup
mg*sintheta=mustatic*mg*costheta
sinthetha=mucostheta
If the block starts sliding, it is equal to the max static frictional force,is this premise correct? We dont consider the kinetic force when the block begins to slide
-->which block will slide fastest?
mg*sintheta-mustatic*mgcostheta=ma
since acceleration is independent, they all slide at the same time. but wouldnt we consider inertia at all in this case?

I think your first premise looks good. It begins sliding when you set theta so that the equation holds with the max static frictional force. Kinetic does not matter until after they begin sliding.

For the second one, you just have to look at your equation:

mg*sintheta-mustatic*mgcostheta=ma

Since mass appears in every term, you can divide the equation by mass (given that it is not dividing by 0) and it should describe the physics that occurs. Since you can divide by m and it completely disappears, it does not matter. This is why those physics questions that ask about how far it takes a car to stop usually don't require knowledge the car's mass, just mu.

Does this make sense or answer your questions?