Force arrows

Using arrows to represent forces

By now you'll have noticed that we use arrows to represent forces. These force arrows (or force vectors as they are referred to in physics books) are very helpful because they can be used to represent the two essential features of any force:

• The direction in which the force acts is shown by the direction of the arrow.
• The size of the force is shown by the length of the arrow (the longer the arrow the bigger the force).

A third convention to be consistent about in drawing force diagrams determines the positioning of force arrows:

• Each arrow is drawn so that it starts from the point where the force acts.

It'll also be helpful to always draw the forces in a particular style (there get to be a lot of arrows on diagrams in physics, not all of them forces). You might also choose colour to show the kind of interaction that the force replaces.

An essential skill is to be able to add the arrows as needed – and not any more than are needed. There are clues in the interactions between the object and its environment that'll help you identify the forces. Later we'll see what those clues are – more on that in episodes 2 and 3.

Adding arrows to a simple situation

Think about the forces acting on a book that is sitting on a table.

The upward push of the table on the book acts on the lower surface of the book and the force arrow is drawn from that lower surface.

The downward pull of the Earth (the gravitational pull of the Earth) on the book is taken as acting through the centre of the book, and the force arrow is drawn from that central point.

Being able to draw arrows to describe forces is an important skill. The activities in the Teaching Approaches section are designed to help pupils practise identifying forces though the use of cut-out card arrows, using these arrows to show forces in a force diagram.