A vector is a quantity that has both size and direction. Force is a vector. A quantity with size only, such as mass, is a scalar. Because force has a direction, we draw it as an arrow. The length of the arrow shows the size of the force and the arrowhead shows the direction it acts in.
A free body force diagram shows one object on its own, drawn as a box or a dot, with an arrow for every force acting on it. Each arrow starts on the object and is labelled with the name of the force. Only forces acting on the object are drawn, not the forces it exerts on other things. For a book resting on a table, the weight arrow points downwards and the normal contact force arrow points upwards. The arrows are the same length because the forces are equal.
Arrows are drawn to a scale. With a scale of 1 cm for every 2 N, a 10 N force is an arrow 5 cm long.
