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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  
with the present punctuation, I'd say that 'acting upon' refers to 'the air'.
I'd agree with Taka -

a force of friction could act upon your skin

air itself does not act upon your skin
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Hi guys,
I agree with Amandine. It's a matter of punctuation.
Either way, the meaning is almost identical.
a force of friction could act upon your skin

air itself does not act upon your skin

Perhaps we are getting away from grammar and into physics, but I'd say that the air acts on your skin and we call the result friction. Only the skin and the air are real things.

Best wishes,
Clive
Clive, what exactlly do you mean by 'punctuation' here? I mean, there is no comma, semmicolon, or anything like that.

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The original writing has this sentence in the same paragraph:
Since our relative surface area is so small at our large size, we are ruled by the forces of gravity acting upon our weight.


Don't you think the author is talking about the forces acting upon our skin?
It's "forces". End of story. Emotion: smile
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Hi,
Well, when you look at the second sentence you added, the writer is clearly talking about forces, so this extra information suggests that was also his intent in the original sentence.

But if you just look at the original sentence, the meaning still seems a bit ambiguous to me and I'm inclined to say that the phrase 'the air acting upon its surface' modifies the nearest noun, which is 'air'. As I said, the meaning either way is not greatly different.

By 'it's a matter of punctuation', I meant that the existing way it's punctuated suggests to me that the phrase refers to 'air'.
It would more clearly refer to 'air' with this punctuation:
....the forces of friction, from the air acting upon its surface, overcome the weak influence of gravity.

Maybe, just maybe, this punctuation might suggest it modifies 'forces':
....the forces of friction from the air, acting upon its surface, overcome the weak influence of gravity.

Best wishes,
Clive
Sorry, Clive. Could you please elaborate this phrase of yours?
the existing way it's punctuated


To my eyes, there is no punctuation exsisting in the original sentence...
Dear Taka,

«friction» is the effect of «movement through air» on the surface of the insect.

It may be rewritten:-

"as the forces of friction, caused by movement through air, overcome the weak influence of gravity"

Kind regards, Emotion: smile
Goldmund
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