Consider the diagram where a tanker is accelerating with acceleration a.
Consider an elementary particle of the fluid of mass dm.
The acting forces on the particle with respect to the tanker are shown above.
Now, balancing forces (as the particle is in equilibrium along the inclined direction component of weight=component of pseudo force `dmg sin theta=dmacos theta` (we have assumed that the surface is inclined at an angle `theta`) where, dma is pseudo force
`implies g sin theta=acos theta`
`implies a=g tan theta`
`implies tan theta=(a)/(g)=` slope
