A force is a push or a pull that can change the motion of an object or do work to change the energy of an object or system of objects.

Sometimes forces behave exactly like we expect them to. In the case of contact forces, two big objects push or pull directly on one another by touching or by being connected by a rope or a spring.

Other times, like when the objects are separated by some distance, or the objects are really small like electrons or atomic nuclei, the forces may behave much differently from what we expect.

Physics offers two explanations of how these forces work—field theory and carrier particle theory. Field forces are better explained by fields, and others are explained better by carrier particles. Unfortunately the two explanations don’t work together—current research is still looking for one explanation that describes the nature of all forces.

What is a Force Field?
One way that scientists explain forces is through the idea of fields. A field is the effect that a particle has on the space around it because of its charge or its mass. The charge or mass of an object changes the space around it so that if you were to put another charge or mass near it, there would be a force. In the following demonstration, you will see the field that results from a strong charge.

The first thing you need to do in this demonstration is to uncheck the boxes for information you don’t need. You should uncheck ‘potential gradient,’ ‘equipotential lines,’ and ‘initial velocity.’ Next, you need to adjust the slider at the top of the right side so that the charge strength is 4. Click to place a charge. The white lines represent the electric field that is caused by that charge. Try each of the following to see how the field behaves:

1. more than one positive charge in the same place
2. two positive charges separated by some distance
3. two negative charges separated by some distance
4. one positive charge and one negative charge separated by some distance

What is a “force carrier particle”?
Well, pretend you want to knock a bird out of a tree 10 meters away. You must exert a force to do this, but the bird is out of your reach. So, you throw a stone at the bird. If you're good enough, you will successfully exert a force on the bird and knock it down from its perch, with the stone being the force carrier.

Particles can exert forces on each other in a similar way. Protons and electrons can exert electrical forces on each other by exchanging a particle called a photon. Depending on the type of photon and the charge of the particle that receives it, the force can be either a push or a pull.

Four basic forces of Nature
All forces fall into one of four categories:

The Forces of Nature in Order of Strength
1. Strong nuclear
2. Electromagnetism
3. Weak nuclear
4. Gravity