3. 5 m A D с B 0.5 m 1.3 m Figure 2 A beam ADCB has length 5 m. The beam lies on a horizontal step with the end A on the step and the end B projecting over the edge of the step. The edge of the step is at the point D where DB = 1.3 m, as shown in Figure 2. When a small boy of mass 30 kg stands on the beam at C, where CB = 0.5 m, the beam is on the point of tilting. The boy is modelled as a particle and the beam is modelled as a uniform rod. (a) Find the mass of the beam. (3) A block of mass X kg is now placed on the beam at A. The block is modelled as a particle. (b) Find the smallest value of X that will enable the boy to stand on the beam at B without the beam tilting. (3) (c) State how you have used the modelling assumption that the block is a particle in your calculations. (1)