Physics forces are external objects or agents that cause change in the motion of other free objects or stress in fixed objects or bodies. Quite simply, a force applies a push or pull to an object that causes it to change direction, change velocity, or deform to some degree. Physics forces have magnitude and direction, which makes them, for mathematical purposes, vector quantities. There are many different types of forces, from simple physical objects colliding to complex electromagnetic fields repelling various objects. Scientists still do not fully understand the workings of forces, as many, such as gravity, are intrinsically linked to the largely-unknown field of quantum physics.
There are many different physics forces that have been observed and that are used in calculations concerning physics. Applied forces are generally considered to be the simplest of forces. Though most kinds of forces could typically be considered applied forces, the term is usually reserved for actions, often human, that directly push or pull on a system. Another common and important force is known as the normal force. The normal force is the equal force applied by an object, such as the floor, when another object applies pressure to it; this force is the reason people do not sink into the ground when they walk across it.
One of the most important forces is gravity, which was not identified as a force until the work of Isaac Newton. Gravity meets the definition of a force because it causes change in the motion of objects. A ball thrown into the air will, of course, return to the ground because of the force of gravity. Electromagnetic forces are also commonly-studied physics forces involving the pulls and repulsions between differently charged objects.
Nuclear forces are an integral part of the study of physics forces, especially in the area of quantum physics. There are two primary types of nuclear forces, the strong nuclear force and the weak nuclear force. The strong nuclear force is the force that is responsible for holding together the subatomic particles that make up the nucleus of an atom, and the weak atomic force is involved in the decay of subatomic particles.
There are many other kinds of forces as well. All physics forces, however, do essentially the same thing in very different ways and on very different scales. They cause change, usually relating to motion, in other objects. Many physics forces are interrelated and can have varying effects on each other.