Types of forces
There is no general formula to allow us to determine the force a given interaction represents. For each type of interaction, there is a different force law.
Applied force
Applied force is the force exerted on an object by a person or another object. There is no force law describing the magnitude of this force; we have to obtain it from the conditions of the interaction, using Newton‘s second law, for instance. When we pull a box across the floor, the force we exert is an applied force; its magnitude can be determined from the fact that we have to overcome friction (see later) to keep the object moving.
Normal force
Normal force is the support force exerted upon an object which is in contact with another stable object. It is always perpendicular to the surface of the stable object. Its magnitude is not fixed; it can be determined from the motion (or lack thereof) of the mobile object.
Weight
Weight is the force with which a suspended or supported object at rest in a gravitational field pulls the suspending object or pushes the support.
\[\mathbf{F}_{\mathrm{W}} = m \mathbf{g},\]
where \(\mathbf{g}\) is the acceleration due to gravity:
\[|\mathbf{g}| = g \approx 9.81\,\frac{\mathrm{m}}{\mathrm{s}^2}\]
Weight is mainly due to gravity, but it also includes forces due to the spinning of the Earth about its axis.
Tension force
Tension force is the force exerted by a string, rope, cable, wire &c when it is pulled tight.
It is directed along the length of the cable and pulls with forces of equal magnitude on opposite ends.
Spring force
The spring force is the force exerted by a compressed or stretched spring.
It is always directed opposite to the compression or stretch.
The magnitude of the spring force is proportional to the stretch or compression:
\[F_{\mathrm{spring}} = - k \Delta x,\]
where \(\Delta x\) is the stretch or compression and \(k\) is called the spring constant or force constant. This is sometimes called Hooke's law.
Several other forces exists, such as
- drag force – refers to forces that oppose the relative motion of an object through a fluid (sometimes called air resistance or fluid resistance)
- electrical force
- magnetic force
- force of gravity
- nuclear forces
- &c.
We shall discuss the forces of friction separately in what follows.