Question

why is it important in a friction experiment of force, that a string pulling a block is parallel to the horizontal surface?
PLEASE HELP???

Answer 1

I would imagine it's because since the string is pulling the block, as long as it is parallel to the horizontal surface there will be no other factors to take into account other than the force that is pulling the block, the blocks weight and gravity, and the friction constant.

If the string was at any other angle (in a 2-D plane), it would either be pulling the block down or pulling it up. Both would affect the way block reacted to the friction.

Answer 2

because if the string is slightly angled up, some of the force pulling on the string to move the block side ways will also go to pulling a slight portion of the block up.