The kilogram, metre and second are all SI base units. A uniform beam AB of length 6.0m is placed on a horizontal surface and then tilted at an angle of 31° to the horizontal, as shown in Fig. 2.1. [Figure 2.1] The beam is held in equilibrium by four forces that all act in the same plane. A force of 90N acts perpendicular to the beam at end A. The weight W of the beam acts at its centre of gravity. A vertical force Y and a horizontal force X both act at end B of the beam.