Question

why does an object float or sink when placed on the surface of water

Answer 1

if the object has more mass than that of the water it will sink if its mass is less than that of water it will float

Answer 2

When the density of a substance is less than that of water, it floats... however when the density of the object is more than that of water..., it sinks.

example - feather floats on water but iron sinks

hope it helps
@PoojaBBSR