An interaction which produces a change in the motion of an object.
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Motivation for Concept
Consider a bowling ball (or some other heavy object). If you want the ball to move, you have to interact with it. If you want the moving ball to turn, you have to interact with it. If you want the ball to stop moving, you have to interact with it. In physics, such interactions are called forces. If you want to move the ball, you will probably have to apply a contact force by using your hands or feet. 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.
Statement of Newton's Laws
Newton's famous Three Laws form the basis of a scientific understanding of force.
First Law
Newton's First Law describes what happens in the absence of forces. If an object is moving with no force acting upon it, then it will move with constant velocity. Note that velocity is a vector, so this statement implies that the object will keep the same speed and the same direction of motion.
Second Law
Newton's Second Law defines force as the time rate of change of momentum:
\begin
[ \vec
\equiv \frac{d\vec{p}}
]\end
If many forces act upon an object, then the change in the object's momentum is equal to the combined effect of all the forces:
\begin
[ \sum_
^{N_{F}} \vec
_
= \frac{d\vec{p}}
] \end
It is important to note that the sum is only over forces that act on the object whose momentum change appears on the right hand side.
In most cases, the object under consideration will have a constant mass. If that is so, then the derivative of the momentum can be rewritten in the traditional formulation of Newton's Second Law:
\begin
[ \sum_
^{N_{F}} \vec
_
= ma ]\end
This form of the equation is the basis for the Point Particle Dynamics model.