How do you tell whether a force acting on an inclined plane is going up or down in its perpendicular component to the plane? 
I'm practicing mechanics, and I had to resolve the following forces perpendicularly to the inclined plane in order to work out the reaction force (plus the weight of the ball)
But I cannot tell whether the 5N force is going up or down when resolved perpendicularly to the inclined pline. According to the book, the answer is up. I thought maybe because the force acts at 45 degrees to the plane, and so therefore if it's 45 degrees or more, then the force has to be acting upwards
However, for this question (the angle of the plane to the horizontal is 30 degrees, it's not very clear in the picture)

the 20N force acts downwards in the component perpendicular to the plane, whereas if I use my aforementioned reasoning, I would expect it to act upwards, since it would have to be more than 45 degrees to the plane to act downwards.
So I don't really understand how I am to determine whether a force's perpendicular component to a plane is up or down when it is acting on an inclined object, and my attempts to do so have contradictions.
 A: Definition:  The "tail" of a vector is the end that is straight (no arrow).
Definition:  The "head" of a vector is the end with the arrowhead.
For each force, draw a right triangle such that:


*

*The original force line is the hypotenuse

*Draw a vector with the tail starting at the tail of your original force, pointing parallel to the plane.

*Draw a vector with the tail starting at the head of the vector from step 2, perpendicular to the plane, ending at the head of the original vector.


The direction of the vector from step 3 gives you your answer.
A: There is a trick that every mechanical engineer uses to solve the problems on mechanics 
firstly draw a perpendicular at the point of application of the force on the plane. 
then make an arrow along this line which you have constructed and along the plane. this arrow will point to the gross direction of the vector . 
like in your first example the arrow on the construction line will be in the upward direction 
but in the second case the arrow direction will be in the downward direction. 
this is a homework question so according to the policy of the cite I can guide and not give the exact solution.
Hopw you can find the hit useful
