Question

why does an object float or sinm when placed on the surface of water. also explain buoanoy?

Answer 1

Objects float when they are positively buoyant, or less dense than the fluid in which they are sitting. Buoyancy or upthrust is an upward force exerted by a fluid that opposes the weight of an immersed object. 

Answer 2

The force exerted by any fluid to that object which is immersed in it (either partiality or wholely) is known as byoant force and the tendency to apply byoant force is called byoancy.

An object swim or float on a liquid just because its density is lesser than the fluid density and if it will be greater it will sink.
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