Calculate Average Blood Speed with Volume Flow Rate | Physics Problem Help

[diamond_dust]

Volume Flow Rate, Help?

1. The problem statement, all variable and given/known data

The volume flow rate in an artery supplying the brain is 3.5E-6 m^3/s.

(a.) If the radius of the artery is 5.4E-3 m, determine the average blood speed.
(b.) Find the average blood speed at a constriction in the artery if the constriction reduces the radius by a factor of 7. Assume that the volume flow rate is the same as in part (a.)

Homework Equations

Q = Volume flow rate = AV

The Attempt at a Solution

I got (a.) but I need help with (b.). I think my radius for part (b.) is wrong, but I don't know how else to get it.

For part (a.) I got 0.0382 m/s by using

Q = Volume flow rate = AV = pi x r^2 x v = 3.5E-6 m^3/s

and using the given radius in part (a.) I solved for v.

For part (b.) I divided the given radius by 7, and subtracted that from the original radius to get a radius of 4.63E-3, and I put it into the equation the same way as part (a.) and I got 0.0520 m/s, but it's wrong. Help?

 

[Redbelly98]

They mean that the constricted radius is 1/7 of the original radius, not 6/7 of it.