Question

why needal is floating in water but not soap water​

Answer 1

The steel needle floats on top of the water because of surface tension. ... The water molecules make an invisible skin on the water's surface that allows things like the needle to float on top of the water. When soap is added to the water, the water molecules aren't drawn together as strongly.

$hope \: it \: helps \: you$

Answer 2

Hey mate...
An iron needle doesn't float if you just drop it into the water. Placing it like that in water and it will sink. This is because the needle is many times denser than water. However, there is a trick to make it float:
Make sure the needle is dry and not soapy.Tear off a square piece of the toilet paper slightly larger than the needle in all directions.Put the needle on top of the paper.Gently lower the paper onto the water.Wait for the paper to sink - you may want to very gently push the edges down to help it.However, if you keep a needle very carefully it will float.

So, why does it float in the first place? 

The answer lies in the surface tension of the water. The cohesive forces between the molecules is responsible for the surface tension. When the molecules on the glass or container do not have a surface to stick to, they cohere more strongly to the molecules they are associated with. Since the molecules on top have no neighbors on the top to cohere to, they associate with their other neighbours more strongly. This creates a strong bond between the top layer of molecules. 



The molecules inward face no net force because they all cancel out each other. However, the surface molecules face a net inward force because there is no force for them on the surface to cancel it. This causes the molecules on the surface to 'contract' and resist being broken or stretched. Hence, the 'tension'.

When a small body like a needle is placed onto the water carefully, the surface tension will not break due to the small mass of the object. Here, Fs>FwFs>Fw. As long as the surface tension is undisturbed, the needle will continue to float. Agitating the water will cause the surface tension to break and hence the needle will then sink.

An incredible image of surface tension of water demonstrated by this picture of a swimmer.

There's a property of any liquid called surface tension. It's the force of attraction between the surface molecules of the liquid.

The molecules inside the liquid have equal cohesive forces from all directions, however the molecules on the surface do not have cohesive forces from the top side (exposed to atmosphere). This creates a net download (or inward) force on the liquid molecules at the surface, and they tend to form a sheet that holds the liquid tightly. That's the reason why a raindrop is spherical (least surface area for a given volume).

The molecules at the surface are held together, to form the imaginery thin sheet, are held together by the surface tension forces.

If the weight of any object is not enough to break this surface tension force, it will not be able to penetrate the top surface, and hence float. In fact it's not floating, it's not even penetrated into water. Floatation occurs due to buoyancy. Here, no buoyancy effect are observed because water is not displaced at all.

Also, as someone said earlier, if dropped from far above the needle may sink, this happens as the impact load is far greater than the surface tension forces. So,

Surface tension causes a needle to float on water.



The steel needle floats on top of the water because of surface tension. ... The water molecules make an invisible skin on the water's surface that allows things like the needle to float on top of the water. When soap is added to the water, the water molecules aren't drawn together as strongly.

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