For example, you might find it impossible to move a 100 kg box no matter how hard you try, because the frictional reaction force is counteracting your pushing action force. We will be covering reaction forces more in Chapter 2.

Force as a vector, units

Force is a vector, meaning it has both magnitude and direction. Therefore it can be split or resolved into its rectangular components in the x- and y- directions, using the sine and cosine functions:

The unit for force is Newtons (N). And yes, this is the guy who discovered gravity by observing a falling apple.

Resultant forces

Most static problems have more than one force acting on a body. It is useful to obtain the resultant force acting on the body, and there are two methods to do this:

Method 1: Addition of rectangular components (recommended)

The resultant of 2 forces in the same direction is just the direct sum of both forces. The idea behind this method is to:

1. Resolve the 2 arbitrary forces into its rectangular components, and add them to get the resultant rectangular components.
2. Then we determine the resultant magnitude and direction.

Method 2: Parallelogram method

This method works by:

1. Drawing a parallelogram on the vector diagram based on the 2 arbitrary forces, and
2. Determining the magnitude and direction of the resultant force using the diagonal of the parallelogram. The sine and cosine rules are useful here.

We’ve covered quite a bit of theory on forces already! Let’s look at an example now.