# Poiseuille's law- Radius vs Flow speed

Discussion in 'MCAT Study Question Q&A' started by IlyaR, 01.13.14.

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1. ### IlyaR 2+ Year Member

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I was going over the wiki premed cards and couldn't decide whether there was an error or if I'm just not understanding something

Should the answer be a exponential, with a curve towards the x axis?

According to the equation,

AV=dP(pi)r^4/constant

V(pi)r^2=dp(pi)r^4/constant

V=dp(r^2)/constant
So...as TBR says...the graph curves towards the axis which displays greater change.

Am I wrong here?

3. ### Lifeman 7+ Year Member

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just graph y=x^4

4. ### Odi

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Doubling the radius causes an increase in flow rate of 16X

If everything is held constant than Q=r^4

The flow rate increases 16x as the radius is increased by a factor of 2x. Flow rate is changing more quickly so D is most reflective of this.

What's the answer on the card?

5. ### IlyaR 2+ Year Member

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Card says d. But what I don't get is why it's bending towards the y axis. R isn't increasing more than v

Last edited: 01.13.14
6. ### Odi

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I see what you're saying. If the axes labels are as is than yeah the graph should resemble x=y^4 and curve to the x axis... not y=x^4 as is.

Looks like they labelled their y and x axis wrong.

IlyaR likes this.
7. ### IlyaR 2+ Year Member

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Cool. Just needed someone to confirm my suspicions lol.