Question

Why does the object floats ir sink when places on the surface of water

Answer 1

When the object has density less than 1 gcm3, then it floats on the surface of water, because it always displaces more weight of water than its own weight. As buoyant force is more than its own weight, therefore, it floats.

When the object has density more than I gcm3, then it sinks in water, because, it always displaces less weight of water than its own weight. As buoyant force is less than its own weight, therefore, it sinks.

Answer 2

$\bf{\underline{\underline{Correct\: Question}}}$

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

$\bf{\underline{\underline{Answer:}}}$

When an object is put in water, then two forces act on it:

(i) weight of the object acting downwards (which tends to pull down the object), and

(ii) buoyant force (or upthrust) acting upwards (which tends to push up the object).

Now, whether an object will float or sink in water will depend on the relative magnitudes of these two forces (weight and buoyant force) acting on the object in opposite directions.

(a) If the buoyant force (or upthrust) exerted by water is equal to or greater than the weight of the object, the object will tloat in water.

(b) If the buoyant force (or upthrust) exerted by water is less than the weight of the object, the object will sink.