Why Does the Ramp Exert Total Force mg Instead of mgcos30 on the Mass?

[Lavace]

My exam is in an hour, so quick replies are dearly needed!

http://www.ph.qmul.ac.uk/phy116/Exams/Mid%20Term%20Test%202006%20Answers.pdf
Question 3)a

I would have said the force exerted by the RAMP on the MASS is mgcos30, not mg as my lecturer has said (and it's the same in every paper).

What am I not seeing here? The only thing I can think of is due to the axis of where F is.

Thanks!

 

[diazona]

The block is not moving, thus a = 0. Also, gravity pulls down on the block with a force equal to mg. If you accept these two facts, balance the forces in the vertical direction and see what you get.

 

[Redbelly98]

I just saw this post, too late for the exam, but I have a comment that may help.

mg cos30 is the normal force exerted by the ramp on the block. But the question is really asking for the total force F exerted by the ramp, which includes the friction and normal forces.

As diazona said, F must balance against the downward, vertical force mg.