Air resistance and time of flight

[Trayshawn]

I read a few old threads on this but none seemed to come to a satisfying answer.

How does air resistance affect time of flight?

When you throw a ball up, air resistance DECREASES the time necessary to reach the apex (since the apex is lowered). But when the ball falls down, air resistance INCREASES the time necessary to hit the ground. The opposite effects make the result ambiguous. But, to my surprise, I've found that both EK AND TBR have asked it..

As you might imagine, this also extends to questions of how air resistance affects range. It decreases horizontal velocity, but has an ambiguous affect on time thus making the affect on range ambiguous as well.

I sware I HATE when things like this come up.

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[milski]

Yes, you won't get a definitive answer because in general it depends and you need more information.

With a few exceptions, like something dropped with no horizontal speed at all, you will have to do the math to find out which of the two opposing factors prevails.

 

[Trayshawn]

you would think so right? but examkrackers 1001 physics #123 and #125 and #96 doesn't seem to agree. Neither does a question TBR physics chapter 1 passage 2.

 

[Trayshawn]

Hmm...actually I just read the answer to #96 and "Time is trickier since air resistance decreases the trip upward, but increases the trip downward. Since air resistance has a greater affect on faster moving bodies, teh trip upward is decreased by more than the trip downward is increased."

I can live with that.

 

[milski]

Hmm...actually I just read the answer to #96 and "Time is trickier since air resistance decreases the trip upward, but increases the trip downward. Since air resistance has a greater affect on faster moving bodies, teh trip upward is decreased by more than the trip downward is increased."

I can live with that.

That's all good but it assumes that the initial speed up is significantly lower than the terminal speed for that body and that the fall is for a much longer distance than the raise. It might be true in a lot of practical situations but I can come with counter examples. I'm not sure why they are drilling so much on a subject which is so lightly treated in intro physics anyway.

 

[Trayshawn]

yea i feel u.

i dont think they are assuming that the fall is a much longer distance than the rise though. If that were the case, then even though the air resistance going up would be greater, the air resistance going down would act over a longer distance and likely have the overall greater effect on time of flight.

Also not really sure how terminal velocity fits into the picture.