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.
Third Law
Newton's Third Law is a rule for determining the effects of forces. This Law states that whenever one object applies a force on a second object, the second object must apply a force of the same size but opposite direction on the original object.
This law is often misunderstood. The wording makes it seem that forces are always a choice, but this is certainly incorrect. Objects apply forces without choosing to all the time. This law is simply describing well known consequences of action. Trying to change the motion of a bowling ball with a swift kick is a dangerous idea, because the bowling ball will automatically push back on your foot. Trying to tackle a 200 pound athlete running at full speed is going to hurt, even if they do not purposely push against you.
Types of Forces
Newton's Laws describe the consequences of forces and give the rules they must obey, but the laws do not explain the types of forces that can be exerted. There are a vast array of ways for objects to interact with each other, but the ways that are commonly treated in introductory physics courses is a rather short list:
- contact forces occur when one rigid body comes in contact with another.
- [gravity] is the attraction at a distance between massive objects. In introductory physics, we most often consider the force of gravity exerted by the earth on objects near its surface, in which special case the force is usually called weight.
- normal forces are a special case of contact force when an object is moving along a surface like a floor, ceiling or wall. The normal force is the portion of the contact force applied to the object by the surface that is directed perpendicular to the plane of the surface.
- tension is a force exerted by a string or rope.
- friction is a force exerted by a surface on an object moving along (or at rest on) that surface that is directed parallel to the plane of the surface.