why does water not bounce back after striking a smooth solid surface while a ball does so?

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valentin68 | College Teacher | (Level 3) Associate Educator

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Water is a liquid and the ball is solid. Interaction forces between molecules of liquids are much smaller compared to interaction forces between solid molecules. Because of this, liquids have their own volume and no particular shape, while solids have both their own volume and shape. Having a constant shape and volume, implies that when a force acts on particular component (let say for simplicity on one molecule) of solid, the same force is transmitted to all molecules (because each molecule is pinned in its place by already existent very high interaction forces). By contrast, in a liquid, only the pressure (which is a mediated force on to a certain surface) not the force on a single component, is transmitted equal in all parts of it.

In other words when a drop of water collides with a surface the cohesion forces between water molecules are so small that the force of the impact is not transmitted equally and entirely in the total volume of the drop.

Observation:

The behavior of the water drops, depends also on the adhesion forces between water molecules and surface on which it bounces. Usually water wets surfaces (the adhesion forces are greater than the cohesion forces) and this also explains why water does not bounce back on normal surfaces (it sticks to the solid surface). However there are hydrophobic surfaces for water for which the adhesion forces are so small that makes a water drop bounce back. In this case the fraction of the force that is transmitted in the volume of the drop (by cohesion forces) is enough to make it bounce a bit from the surface.

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