# How does surface tension prevent a pin from sinking?

Please explain simply how surface tension in water which as far as I understand is a force pulling the water molecules at the surface towards each other horizontally so how does that prevent a vertical gravitational force downwards , also the water molecules at the surface are forced downwards because the there is no water molecule on top to pull it so that should make it even more possible for things to sink like a pin or something. Why do people keep saying it’s the surface tension keeping it from sinking like even the contact force makes more sense than surface tension.

• Imagine walking on a stretched trampoline. Jan 16, 2021 at 12:19

Surface tension is caused by the attraction of like molecules to each other (cohesion). Water has a higher surface tension than many other liquids because of a relatively high attraction of water molecules to each other due to the hydrogen bonds. A small lightweight object may not be heavy enough to push through the "skin" of the surface tension of water as it would have to push water molecules away from each other against their cohesive attraction to sink between them.

This is how surface tension is defined:

Surface Tension is the the force per unit length in the plane of the liquid surface, acting at right angles on either side of an imaginary line drawn in that surface.

According to the Wikipedia,

Surface tension is the tendency of liquid surfaces to shrink into the minimum surface area possible.

So, it's obvious that to acquire the minimum surface area possible, liquid molecules should have less space inbetween them. Water molecules have a special kind of bond inbetween them called the hydrogen bond which causes the water molecules to stay together. Due to this hydrogen bonding, water has a greater cohesive force i.e., a water molecule shows more attraction towards another water molecule rather than an iron molecule(of the needle).

An iron (or whatever the substance be) needle is indeed denser than water and hence will drown due to its weight. But this property of water keeps it from drowning.
So, when you place a needle on the water surface, the water molecules in contact with the needle feels that it's better to stay close to one another than to attach to a bloody needle, and they form a kind of skin as mentioned in the other answer. On the other hand, if you don't put the needle softly, it may pierce this skin of water molecules and drown.

There is an energetic cost to having a surface between two interfaces. Increasing the surface by $$\Delta$$ costs $$\Delta W=\gamma \Delta A$$, where $$\gamma$$ is the surface tension. We can imagine displacing our floating needle downward a little bit by $$\Delta h$$. This will cause the potential energy to decrease by $$-mg\Delta h^\dagger$$, but the surface of the water gets larger, which increases the energy by $$\gamma \Delta A$$. For water, the surface tension is particularly high and it is possible for the increase in surface energy to overpower the energy gain by sinking the needle. In that case, the needle will float.

$$\dagger$$ Technically, $$m$$ is here the mass of the ball minus the mass of the displace volume of water.