Example 7.5b Physics Berkeley

[JP2740]

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Which single change makes it easiest (i.e. creates the smallest pressure difference between your mouth and atmosphere) to drink a milkshake?

I know some of you guys have the book so I'll refrain from posting the choices. But I thought it was "melting the shake until its viscosity drops by 50%, but keeping the flow rate and straw fixed" using 50% drop for n, but I guess n is coefficient of viscosity and not exactly viscosity. But how do they relate?

They said, "Drinking the shake at 25% of the usual flow rate, without changing the straw or shake" - this seems to me as less of a drop. I don't know where I'm going wrong.

 

[JP2740]

Might as well help with 7.9b and the membranes and surface tension... I actually picked the completely wrong answer, C lol. Is it because, the surface tension of the membrane tends to hold it stiff, and since the pressure on the right side + membrane losing surface tension allows it to be blown out? I thought surface tension increased just because the look of how it turned out but i guess thats wrong.

 

[BerkReviewTeach]

Which single change makes it easiest (i.e. creates the smallest pressure difference between your mouth and atmosphere) to drink a milkshake?

I know some of you guys have the book so I'll refrain from posting the choices. But I thought it was "melting the shake until its viscosity drops by 50%, but keeping the flow rate and straw fixed" using 50% drop for n, but I guess n is coefficient of viscosity and not exactly viscosity. But how do they relate?

They said, "Drinking the shake at 25% of the usual flow rate, without changing the straw or shake" - this seems to me as less of a drop. I don't know where I'm going wrong.

"Easiest to drink" would imply needing to generate the smallest pressure difference between the top and bottom of the straw to get it to flow at some desired flow rate.

Choice A talks about using a longer straw, which would require a greater pressure difference, because as L increases deltaP would have to increase by the same amount to keep the shake flowing at the same rate. L is found in the denominator of Poiseuille's law, while delta P is found in the numerator of Poiseuille's law. Choice A would make it harder to drink the milkshake.

Choice B talks about using a thinner straw, which would require a greater pressure difference, because as r decreases deltaP would have to increase by the same amount to keep the shake flowing at the same rate. r^4 is found in the numerator of Poiseuille's law as is delta P, so if one term increases, the other must decrease. Choice B would make it harder to drink the milkshake.

Both of these choices should be intuitive too, so there is likely no issue to this point. I think from this point it could be an issue with mis-interpreting the question, which is easy to do. Choice C involves a decrease by a factor of 50% while choice D involves a decrease by a factor of 25%, not a 25% drop.

Choice C talks about reducing the shake's viscosity. To keep the flow rate fixed while halving the shake’s viscosity would require half the pressure difference across the straw, so choice C is possible.

Choice D talks about reducing the flow rate to 25% of its original value without changing the straw or milkshake. To reduce the flow rate by a factor of four while keeping other factors constant would require one-fourth the pressure difference across the straw.

 

[BerkReviewTeach]

Might as well help with 7.9b and the membranes and surface tension... I actually picked the completely wrong answer, C lol. Is it because, the surface tension of the membrane tends to hold it stiff, and since the pressure on the right side + membrane losing surface tension allows it to be blown out? I thought surface tension increased just because the look of how it turned out but i guess thats wrong.

In the picture, they show a slight distortion of the membrane to left initially followed by a large distortion of membrane to the left following the experiment.

The membrane is being forced to the left (volume on the right is increasing while volume on the left is decreasing), so pressure on the right side must have increased relative to the pressure on the left side of the membrane. Choices C and D state the opposite, so they are eliminated.

After the distortion, the membrane is longer, so it must have been stretched in the process. Because the molecules of membrane are being stretched apart, they are farther away from one another, so they are feeling less of an attractive force towards one another. This means that the membrane’s surface tension must have decreased, making choice B a better explanation than choice A.

 

[JP2740]

"Easiest to drink" would imply needing to generate the smallest pressure difference between the top and bottom of the straw to get it to flow at some desired flow rate.

Choice A talks about using a longer straw, which would require a greater pressure difference, because as L increases deltaP would have to increase by the same amount to keep the shake flowing at the same rate. L is found in the denominator of Poiseuille's law, while delta P is found in the numerator of Poiseuille's law. Choice A would make it harder to drink the milkshake.

Choice B talks about using a thinner straw, which would require a greater pressure difference, because as r decreases deltaP would have to increase by the same amount to keep the shake flowing at the same rate. r^4 is found in the numerator of Poiseuille's law as is delta P, so if one term increases, the other must decrease. Choice B would make it harder to drink the milkshake.

Both of these choices should be intuitive too, so there is likely no issue to this point. I think from this point it could be an issue with mis-interpreting the question, which is easy to do. Choice C involves a decrease by a factor of 50% while choice D involves a decrease by a factor of 25%, not a 25% drop.

Choice C talks about reducing the shake's viscosity. To keep the flow rate fixed while halving the shake's viscosity would require half the pressure difference across the straw, so choice C is possible.

Choice D talks about reducing the flow rate to 25% of its original value without changing the straw or milkshake. To reduce the flow rate by a factor of four while keeping other factors constant would require one-fourth the pressure difference across the straw.

NVM my mistake