An insect performs no miracle in walking up a wall or uopn the surface of a pond; the small force of gravity pulling it down or under is easily overcome by the surface forces which act to keep it in position. Throw an insect off the roof and it floats gently down as the forces of friction from the air acting upon its surface overcome the weak influence of gravity.

Which word does 'acting upon' modify' here?

I think it's 'the forces (of friction)', but my book says it's 'the air'...
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Comments  (Page 3) 
OK. Now I finally understand what you meant.

Thank you!