Question

the water following over stones in a stream or river makes them round and smooth mainly due to which friction?

Answer 1

In general, there are four types of friction:

1. Static friction (objects at rest)

2. Sliding friction (objects sliding on a surface)

3. Rolling friction (Objects rolling on a surface)

4. Fluid friction (objects moving in fluids)

The type of friction here is "Fluid Friction".

Fluid friction acts on objects in fluids (liquids or gases). Here, the stones are placed in water, therefore they are in a fluid. The friction is the resistance of the water against the stones, which affects the shape of the stone as it keeps "hitting it".

Answer 2

Water and sand plays a greater part in making stones in a stream or a river into round and smooth.

Also, stones in water or streams in particular gets round due to the plummeting effect which it has to encounter due to flow of water.

The friction which makes the stones round and smooth is called fluid friction.