Direction of a tension in a string
In all pulley diagrams some consideration must be given to the direction in which force arrows are drawn on the strings.
Take the case of a 2 kg mass supported by a string held in the hand (Fig. 8.5). Assuming g = 10m/s2 the tension in the string is 20 N and acts equally in both directions. The arrow pointing upwards represents the force which the string exerts on the load, while the arrow pointing downwards shows the force exerted by the string on the hand. This is another example of Newton’s third law, “action and reaction are equal and opposite” (page 15). In general, we do not put in’ both arrows, but only the one which gives the direction of the force in which we are interested. Thus in Fig. 8.4 both arrows are drawn upwards as we wish to indicate the force exerted by the string on the load. We are not here concerned with the downward pull of the string on the support.