# why does a small pebble sink in water

hey buddy!! what class are u in? actually,the principle of buoyancy kicks in to make the boat float. Proportional to the density of the fluid,it exerts an. hey buddy!! what class are u in? actually,the principle of buoyancy kicks in to make the boat float. Proportional to the density of the fluid,it exerts an opposing force/area(pressure) on any object put in it. for any object to float,the force exerted by the liquid should exceed the weight of the object. now,more the area for the constant weight,more is the force. therefore the boat,a hollow and sheet like structure floats and pebble sinks.

Submarines also work on the same principle;the have these tanks,probably called ballast tanks or something. when they get filled(obviously with water),the weight/area(read density)increases and it sinks.

When it opens the ballast tanks,water is pumped out and sub floats over water. Psst. ,air is also a fluid. this means if u can find a material light enough,it will float(parachutes apply the principle of buoyancy in part). Probably, Mylar will be one such material.

The pebbles because silt and clay settles so slowly through water that some tiny particles take centuries to reach the bottom when pebbles go straight down.

You have to make a couple of assumptions here! 1) The material forming the sand and the pebble has roughly the same density 2) They are both sinking through the same fluid This is because the maximum sinking velocity (Vs) is controlled by Stokes' law: V s = (( g x R 2 x (Dp - Df)) / (18 x n)) Where g = gravitational acceleration R = particle radius Dp = particle density Df = fluid density n = fluid viscosity From the above it follows that if the density of the particles is the same and the fluid through which they are falling is the same then the only other variable is the radius.

As this is on the top of the fraction, the larger it is, the larger the particles velocity.

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