## Section6.6Common Forces

Intuitively, a force is a push or a pull an object exerts on another object. When you push on a wall, you are aware of the fact that you are applying a force on the wall. What is not obvious is that, when you push on the wall, another force is generated simultaneously inside the wall that pushes back on you.

The wall has “reacted” to your push. It has happened because when you push on the wall, you compress atoms of the wall ever so slightly closer, which makes the wall act like a compressed spring resulting in a push back on you. Every force between two bodies has this dual aspect - they act on both bodies at the same time.

Some forces are attractive and others are repulsive. There is no such thing as a neutral force. When you puch on the wall, the force between you and the wall, is repulsive since its tendency is to push the bodies (i.e., you and the wall) apart. Gravity is an example of an attractive force since it tends to pull the two bodies together. Sometimes, it is not so easy to characterize a force as always attractive or always repulsive; for instance, spring force is attractive when spring is stretched and repulsive when compressed as compared to its resting state.

There are four fundamental forces in nature - gravitational, elctromagnetic, weak-nuclear, and strong-nuclear forces. But, in everyday life, we encounter many effective forces that have their origins in gravitational and elctromagnetic forces. We will mostly study the following forces in this chapter. After this list of forces, each is described in some detail. You can come back to them as you need them.

1. Weight
2. Normal
3. Static Friction
4. Sliding/Kinetic Friction
5. Rolling Friction
6. Fluid Drag
7. Spring Force
8. Tension Force

We will be working in the SI system of units, in which the unit for force is Newton, abbreviated by capital letter $\text{N}\text{.}$ How big is a $1 \text{ N}$ force? Using the second law of Newton, we can say that $1 \text{ N}$ force on a $1 \text{ kg}$ object will cause the object to accelerate at $1\text{ m/s}^2\text{.}$