Surface force, or surface tension, is the upward force that the surface of a liquid exerts on an object. It allows small insects to walk on water and makes water sports possible. Surface force occurs because of the molecules at the surface of a liquid. Adding soap or increasing temperature can decrease surface strength. Surface force is a balanced force that allows objects to float.
Surface force, more commonly called surface tension, is the only upward force that the surface of a liquid can exert on an object. This force is the reason small insects, such as water hawks, appear to walk on water. It also makes sports like surfing and water skiing possible. Surface force occurs because the molecules or tiny particles that make up a liquid tend to lock onto each other at the surface, forming a thin layer of slightly higher density than the rest of the liquid.
The surface tension or surface force of water is strong enough to push objects up, allowing them to float. This can be observed by carefully placing a small metal paper clip on the surface of a glass of water, filled to the brim. Since the density of the paper clip is greater than the density of water, it might be expected to sink. With a little care in placement and some knowledge of surface strength, however, this won’t happen. If the paper clip is dropped onto the surface so that the end pierces through the invisible barrier, it will sink, so be careful when placing the paper clip.
A number of factors can decrease the surface strength of water, such as an increase in temperature. As the water temperature increases, the surface strength decreases. Adding soap to water can also reduce surface tension. Soap breaks the grip that water molecules have on each other, allowing water to more easily come into contact with items like dirty dishes. Adding a single drop of soap to a glass of water with a floating paper clip will instantly cause the object to sink and is a successful conclusion to the previously mentioned experiment.
The reason why surface tension can be called surface force is explained by the physical definition of the word force. In physics, a force is defined as a push or pull exerted on an object. When a paper clip is placed on water, gravity pulls the paper clip down. When the surface tension of water is strong enough, however, the water can push the paper clip up with the same amount of force that gravity pushes it down, allowing the object to float. This is an example of balanced forces: the paper clip doesn’t sink or pop out of the surface, it just stays there.
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