Surface force, more commonly referred to as surface tension, is the unique upward push that the surface of a liquid can exert on an object. This force is the reason tiny insects, such as water striders, seem to walk on water. It also makes sports such as surfing and water skiing possible. Surface force occurs because the molecules or tiny particles that make up a liquid tend to lock onto one another at the surface, making a thin layer that is slightly higher in 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 top. 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 force, however, this will not happen. If the paper clip is dropped onto the surface so the end punches through the invisible barrier, it will sink, so take care when placing the paper clip.
A number of factors can decrease the surface force of water, such as an increase in temperature. As the temperature of water increases, the surface force decreases. Adding soap to the water may also decrease the surface tension. Soap breaks the hold that water molecules have on one another, allowing the water to more easily come in contact with objects such as 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 an affirmative end to the experiment mentioned earlier.
The reason 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, though, the water can push the paper clip up with the same amount of force that gravity is pulling it down, allowing the object to float. This is an example of balanced forces — the paper clip neither sinks nor shoots off of the surface, but simply rests there.