What is the main (one) force driving filtration in the glomerular capillaries?

Answer 1

Glomerulus drainage into efferent arterioles. I can't phrase it better than the Wiki page so here's the quote:

"The resistance of the arterioles results in high pressure in the glomerulus aiding the process of ultrafiltration where fluids and soluble materials in the blood are forced out of the capillaries and into Bowman's space."

This pressure differential forces small molecules to pass freely into the Bowman's Space (e.g. water, glucose, salt, amino acids, and urea).

I'll include the Wiki links below. Makes for good reading. You really have to picture it in your head to have it make sense.

Good luck.