Question

why is it difficult for a diver to stay neutrally buoyant in one fixed position?

Answer 1

It can be explained by using the concept of forces. We know that According to Newton's second law Force F can be expressed as the product of mass m and acceleration.

F=ma.....(1)

The above equation states for being still a=0. And the resulting force acting on the system must be zero. But in case of Buoyancy, Archimedes Principle states that any object will float only if buoyant force B balances the weight of the object.

Where, the B can be expressed as

Here, Delta P is the pressure difference and A is the cross-sectional area. Considering all the three equations simultaneously for being neutrally buoyant at one position the net force must be zero which means

If the above relation holds then the neutral buoyancy will occur at one point. But inside the water the pressure difference kept on changing due to wave motion so it will be difficult to stay still at one point. That is why in Scuba diving and in snorkeling one can not be neutrally buoyant. It may be comparatively less difficult in swimming pool rather than in ocean.

In general terms nuetral buoyancy defines the conditions of weightlessness.  That means fulfillment of Eq.(4). Sometimes to achieve this some divers put swim suits/ specially designed suits. People with more fat tend to achieve nuetral buoyancy with less difficulty than those with muscular body.