Clarification about friction equation

[rash219]

Clarification about friction equation !

Homework Statement

Suppose there is an object on the top of a ramp at a rest would these equation/s be correct to preform calculations considering friction applies

$$\Sigma$$F_y = n - (m * g * Cos$$\Theta$$) = 0
$$\Sigma$$F_x = (m * g * Sin$$\Theta$$) - ($$\mu$$_s * n) = 0

Suppose there is an object on the top of a ramp was sliding downwards would these equation/s be correct to preform calculations considering friction applies

$$\Sigma$$F_y = n - (m * g * Cos$$\Theta$$) = 0
$$\Sigma$$F_x = (m * g * Sin$$\Theta$$) - ($$\mu$$_s * n) = (m * a) or
$$\Sigma$$F_x = (m * g * Sin$$\Theta$$) - ($$\mu$$_k * n) = (m * a)

Note : ($$\mu$$_k * n) would be located at ($$\mu$$_s * n)


http://img40.imagevenue.com/loc876/th_28739_dig_122_876lo.jpg


My Question is are these formulas correctly represented so that i could solve any question that involves static and kinetic friction

 

[Dick]

Yep. That's pretty much it. Except you can't consider static friction when an object is accelerating, better put a=0 in that case, as you did the first paragraph. Good luck with actually solving problems!

 

[rash219]

Thanks ! Understood...

 

[Doc Al]

Careful with setting static friction equal to $\mu_s N$. That's the maximum value of static friction--the actual value may well be less.