Solving Sliding Object Down Ramp: Horizontal Forces Only?

[ItsNoah]

I went to ask my teacher for extra help and we were doing a problem about an object sliding down a ramp. When I calculated the Fnet I added all the forces together acting on the box but she said that you only take into account the horizontal forces. Any clarification would be much appreciated!

 

[ItsNoah]

The reason I ask this is because I thought when adding Fnet up you add up all the forces. ALSO: i was trying to calculate acceleration so is this why you only add up the horizontal forces?

 

[PotentialE]

If the box is sliding down the ramp, then the positive forces would be the ones that are making it move, meaning the ones acting downward. The negative forces would be the ones acting opposite of motion, like friction. so when you sum the forces, make sure you have friction and any force applied upward as negatives, and then the force of the plane and any forces applied downward as positives.