Mechanical Principal Question

http://www.mymathforum.com/

I'm sorry but I don't know the answer to this question.
I would like to suggest that you post this question in a more advanced forum.
the link above is one such forum.
It has high school questions but it also has uni questions.
The questions are divided up into different sections
Try to find the most appropriate section in order to maximize your chances of a good answer.

If you get an answer I would be interested in seeing it.

This is a vector math problem. Weight of 3 kg is directly down towards the ground.

a. The normal force is the force 90 degrees to the plane. Since the plane is at 40 degrees to the ground, the angle between the weight vector and the normal force is 50 degrees. From vector math, the normal force is 3 kg * cos(50) = 1.928 kg

b. The box is sitting still, so the force from friction keeping the box from moving down the slope is exactly equal, in the opposite direction, to the force of gravity trying to push the box down the slope. Since the plane is at 40 degrees to the ground, the force pointing down the slope is 3 kg * sin(50) = 2.298 kg. Since you want the horizontal component of this force, it is 2.298 kg * cos(40) = 1.760 kg

I wish I could draw the pictures. It has been a while since I did this sort for thing. I hope some one will check me.