Missing details: figure of the problem is attached.
We can solve the exercise by using Poiseuille's law. It says that, for a fluid in laminar flow inside a closed pipe,
![\Delta P = \frac{8 \mu L Q}{\pi r^4}](https://tex.z-dn.net/?f=%5CDelta%20P%20%3D%20%20%5Cfrac%7B8%20%5Cmu%20L%20Q%7D%7B%5Cpi%20r%5E4%7D%20)
where:
is the pressure difference between the two ends
is viscosity of the fluid
L is the length of the pipe
is the volumetric flow rate, with
being the section of the tube and
the velocity of the fluid
r is the radius of the pipe.
We can apply this law to the needle, and then calculating the pressure difference between point P and the end of the needle. For our problem, we have:
is the dynamic water viscosity at ![20^{\circ}](https://tex.z-dn.net/?f=20%5E%7B%5Ccirc%7D)
L=4.0 cm=0.04 m
![Q=Av=\pi r^2 v= \pi (1 \cdot 10^{-3}m)^2 \cdot 10 m/s =3.14 \cdot 10^{-5} m^3/s](https://tex.z-dn.net/?f=Q%3DAv%3D%5Cpi%20r%5E2%20v%3D%20%5Cpi%20%281%20%5Ccdot%2010%5E%7B-3%7Dm%29%5E2%20%5Ccdot%2010%20m%2Fs%20%3D3.14%20%5Ccdot%2010%5E%7B-5%7D%20m%5E3%2Fs)
and r=1 mm=0.001 m
Using these data in the formula, we get:
![\Delta P = 3200 Pa](https://tex.z-dn.net/?f=%5CDelta%20P%20%3D%203200%20Pa)
However, this is the pressure difference between point P and the end of the needle. But the end of the needle is at atmosphere pressure, and therefore the gauge pressure (which has zero-reference against atmosphere pressure) at point P is exactly 3200 Pa.