Recent content by Fredrik

I keep running into apparent contradictions that undoubtedly just highlight that I still don't understand the basics. Here's one of them: Isn't there a streamline that starts in the person's lungs, goes through the straw and continues in a straight line until the velocity has dropped to zero...

Your plots are interesting, especially the velocity vector plot. I'm curious what assumptions you fed into the software. In particular, have you (or the software) calculated that the stream will have lower pressure, or is that somehow part of the initial conditions?

I know that's what you meant. I just don't understand why the water pressure at the exit is equal to the air pressure. The average speed of the N2 molecules in the air is 530 m/s. The speed of sound is only 340 m/s. My issue with the (probably correct) narrative that we're somehow blowing away...

OK, so now you agree with what I read at the HyperPhysics web site. Can you (or someone) explain to me why the pressure at the exit must be equal to the ambient air pressure? Reminder: We're talking about the picture you can see here, in the section "Pressure drop with length"...

I'm not sure this argument works. Maybe it does if the hammer is super-fast. But if its speed isn't much faster than the average speed of the tennis balls, then I think it would cause an increase in pressure before the drop in pressure. The average speed of N2 molecules in the air is over 500...

I have tried to understand the two cans problem by considering a bunch of tennis balls shot towards the space between two trash cans. If some of the balls hit the trash cans, it would tend to push the cans apart. What makes this scenario very different from the one in post #1 is that here the...

What kind of thing is a low pressure stream of air? Shouldn't it implode like low pressure things in a high pressure environment do?

I don't understand this answer. How can friction alone make the pressure drop at the exact rate (Pascals per meter) required to make the water pressure equal to the air pressure at the exit? When you said "friction", did you mean the force on the water from the outside air? I would have guessed...

I thought of that explanation too, and believed it to be correct for a couple of weeks. But about a week before I started this thread, I dismissed it as incorrect because if it's correct, then it's impossible to create a high pressure stream of air. (Applied to a high pressure stream, the...

This too seems to be inconsistent with what that HyperPhysics page is saying. It claims that the water pressure at the exit will be equal to the air pressure outside. That wouldn't always be the case if the pressure drop is caused by friction.

I would appreciate some help with some basic stuff. Consider this picture, from HyperPhysics. Edit: The image looks fine when I preview. You should be able to see it if you click "Reply". This is about water, not air. They're saying that the pressure is dropping through the horizontal pipe...

Thanks for the input. I haven't had a lot of free time myself these past few days. If I had, I would have responded sooner. I'm with you this far. This is roughly what I had been able to understand on my own, but your comments helped me understand it a bit better. I didn't understand this the...

Thanks for the input, both of you. I believe that this part is correct, but the stuff you said before doesn't really explain the low pressure, I think. But I think I found a different argument for it. See below. This was useful. I found a post where boneh3ad explained how to calculate the...

Hey guys. I have a simple-looking physics problem that I don't really understand, and I remembered this site, where I used to hang out a lot in the beforetime, the long-long ago. (That's a South Park reference). This is the problem: A person blows through a straw between two empty soda cans. Do...

You didn't state the problem accurately, but if it's asking you to determine v(0.02), then all you have to do is to replace the x on the right by 0.02 and perform the computations. To "solve for v" would be to find an equivalent equation with v alone on one side, but the equation is already of...