Force produces a change in the momentum of a mass on which it acts, according to F=ma (Newton's Second Law). Forces result from various types of physical interactions, which always generate a pair of opposite forces acting on two different objects (Newton's Third Law). |
Consider a bowling ball (or some other heavy object that moves with little resistance). If you want a stationary ball to move, you have to exert a force on it in the direction you want it to move, which will accelerate it. If you want the moving ball to turn, you have to exert a force on it toward the side you want it to turn toward. If you want the ball to stop moving, you have to exert a force opposite to its velocity. To change the motion of the bowling ball, you will probably apply a force by using your hands or feet or some object you push against the ball. There are other kinds of forces, however. The earth, for example, can alter the ball's motion through the invisible action-at-a-distance of gravity, often represented as a gravitational field acting on the body at the site of the body.
Newton's famous Three Laws of Motion together comprise his definition of force.
There are many ways to classify forces. For the purposes of the modeling approach to physics, the most important classifications to understand are Internal vs. External and Conservative vs. Non-Conservative. Another commonly encountered classification of forces is by their status as "fundamental" vs. phenomenological.