# A 4He fluid with a mass

1. Nov 27, 2017

### MMS

Hello,

I have two questions regarding a setup concerning fluid 4He.

Assume there's a container of some height filled with liquid 4He and a mass that is released from rest at its surface. Two questions:

(1) Using what technique can it be decided whether the liquid is a superfluid or normal fluid?

(2) Assuming that the liquid is a superfluid, How can I determine the critical velocity?

(1) If we assume that we know the speed of the mass, then if it is less than ~60 m/s (roton minimum), the mass will sink without any friction and so it is a superfluid. However, if its speed is higher, dissipation is created and so it is a normal fluid.

(2) Seems kind of simplistic but: Since it is a superfluid, the only force the mass feels while sinking is g. So the velocity it has once it reaches the bottom is v=sqrt(2*g*h). We increase the height each time, check this relation and compare it to the experimental velocity found. Once there is a distinct mismatch between the two, it means that there was dissipation in the sinking process (the mass now feels another force) and hence we reached v_c.