physics incline questions

Question

A 16kg box is accelerating down a 35° incline. There is kinetic friction acting on the box. Select the correct net equation for the y direction in this scenario.
Choose one answer.
a. Fn-mg=0
b. Fn-mgcos35=0
c. Fn-Fk=0
d. Fn-mgsin35=ma

 

which would be the correct answer? Any guidance would be appericated, thank you! :)

Answer 1

I assume the y-axis is perpendicular to the surface of the plane.  Since there is no acceleration in the y-direction, you can set up equilibrium of forces in the y-direction.  The force of friction is parallel to the surface of the plane, so it does not contribute to force in the y-direction.  The component of the weight in the y-direction is -mg cosθ.  The normal force F_n is in the +y direction.

 

Equilibrium of forces in the y-direction:

 

F_n - mg cosθ = 0

 

θ = 35°

 

F_n - mg cos(35°) = 0

 

The correct answer is (b).

Answer 2

 

This problem has already been answered today. Many "students" posting the same problem with the optional answers list changed in order.

 

And all of these students are writing the same mistake over and over again: there is no such "Kinetic friction". It is friction force or force of friction.