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An unbalanced force or torque causes acceleration. Force accelerates things in a line, and torque is responsible for rotations. By unbalanced we mean that the vector sum of all the forces (or torques) acting on a body is not zero. In plain English, if there is more force pushing to the right than to the left, the object is going to move to the right. The vector sum of all the forces acting on a body is called the net force.

So where do forces come from? Physicists have identified four unique fundamental forces. All forces ultimately come from one of these four.

The 4 Forces of nature

Force Range Relative Strength Example
Gravity infinite 1 orbits, weight
Weak Nuclear 10-17 m 1027 beta decay
Electromagnetic infinite 1038 atoms, molecules
Strong Nuclear 10-15 m 1040 nuclear binding

We do not ordinarily encounter nuclear forces in our everyday lives, but the gravitational and electromagnetic forces are everywhere. The gravitational force is what keeps us stuck to this planet, and what keeps our planet in orbit around the Sun. The electromagnetic force encompasses both electrical and magnetic forces. It does much more than stick magnets to your refrigerator. It is the force that binds atoms and molecules together, so it is ultimately responsible for the strength of solid matter. You cannot pull a steel bar apart with your bare hands because you are not as strong as the static electric attraction between the iron atoms, the very same force that attracts your hair to your comb on dry days.

In practice, though, it is not possible to break all forces down into their fundamental sources on the atomic level. Instead, we proceed as if forces come from springs, motors, muscles, or friction.


Boise State University

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Copyright 2000-2001
  James W. Brennan
Selland College of Applied Technology
Boise State University

Last Updated 07/08/04 by JWB