In physics, forces influence objects, causing their motion to change. Contact forces require objects to touch, such as when the tip of a pool stick nudges a cue ball. Noncontact forces, such as magnetism, affect objects without touching. Physicists have found four fundamental forces in nature; other forces seen in everyday life involve the four main ones, although in ways that are not immediately apparent.
Four Fundamental Forces of Nature
Gravity, electromagnetism, and the nuclear strong and weak forces are the four forces that make the universe tick. Gravity is the familiar attraction between objects that have mass, such as the force that holds a piano to the floor underneath, or the tug between the Sun and planets. The strong force, although crucial, is not normally experienced; it is the “glue” that binds subatomic particles into atoms. The weak force also acts inside atoms and is involved in some types of radioactivity. Electromagnetism combines electricity and magnetism; the force that holds bits of paper to a rubbed balloon is related to the force that makes iron stick to a magnet.
Centripetal & Centrifugal Force
If you whirl a tethered ball around in a circle while holding it by a cord, your arm pulls on the cord, which keeps the ball moving. This is centripetal force; without it, the ball would fly away from you in a straight line. The force produced by the ball tugging outward on the rope is called centrifugal force. As the ball twirls around you, the two forces are equal but act in opposite directions. A physicist calls centrifugal force an “effective” force because it only appears to act on objects.
Static and Dynamic Friction
Friction is a contact force between two objects; the stronger the friction, the more an object resists moving when pushed. Static friction is the force between nonmoving objects. For example, a heavy cardboard box is difficult to push on a concrete floor because of the static friction of the cardboard on the floor. If you push hard enough, you overcome the static friction and the box moves. Once you have it moving, however, dynamic friction keeps the box from sliding away on its own; you must keep pushing to move the box across the floor. Surfaces with low friction are slippery, such as when you walk on a wet tile floor in gym shoes.
The Coriolis force acts across the surface of a spinning object; as the Earth turns, for example, wind patterns are different at the equator than they are at higher latitudes. The difference in wind speeds causes storms to form circular patterns, as seen in hurricanes and typhoons. It does not, however, swirl the water in a draining bathtub; the Earth’s Coriolis force is very weak across small distances.
Springs and rubber bands produce forces from the tension in their atoms and molecules. Here, the force increases the more you stretch the object, up to the point where it breaks. Stretching forces obey a relation called Hooke’s Law, where the force is equal to the strength of the spring multiplied by the length of the stretch.