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# Why does capillary action occur?

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According to the United States Geological Survey, capillary action occurs due to the forces of surface tension, cohesion and adhesion. Capillary action is the tendency for water to move with the spaces of a porous material. For example, when a paper towel is placed in a glass of water, the water travels up the paper towel until the force of gravity overpowers the capillary forces.

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Cohesion is the tendency for water molecules to stick together, whereas adhesion is the tendency for water molecules to stick to other substances. When combined with the tendency of water to resist external forces, which is also known as surface tension, water and other liquids climb up the sides of a thin glass vessel, called a capillary tube. HowStuffWorks explains that the diameter of the capillary tube and the ambient temperature affect the distance that the liquid rises.

There are many examples of capillary action in the natural world. According to HowStuffWorks, compact soil contains small spaces between the grains of dirt. Water rises through the soil via capillary action and evaporates upon reaching the surface. Additionally, as explained by USGS, plant roots rely on capillary action to draw water from the ground and transport it to the plant’s trunk, branches, twigs and leaves.

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## Related Questions

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Newton's third law states that for every action there is an equal and opposite reaction. This means that when an object puts a force on another object, the second object exerts the same amount of force against the first object.

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Friction happens when one object slides along another object. It is the force that resists the relative motion between the two surfaces. Friction occurs in the opposite direction of the moving object's motion and reduces its speed.

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Friction is caused by the bumps on the surface of one object colliding with the bumps of another object's surface. All surfaces have bumps, created by their molecular structure. These bumps may only be visible on objects with seemingly smooth surfaces through the use of a microscope.