A metal bar 70cm long and 4.00 kg in mass supported on two knief - edge placed 10 cm from each end. a 6.00 kg weight is suspended at 30 cm from one end. find the reaction at the knife-edge?

First, you make a diagram of all the forces acting on the system. This is shown in the figure. We have to determine F1 and F4. Let's do a momentum balance. Momentum is conserved so the summation of all momentum is equal to zero. Momentum is force*distance.

To determine F1: (reference is F4, so F4=0)

∑Momentum = 0 = - F2 - F3 + F1

0 = (-4 kg) (9.81 m/s2) (0.25m) - (6kg) (9.81 m/s2) (0.5-0.3m) + F1 (0.5-0.1m)

F1 = 53.96 N (left knife-edge)

To determine F4: (reference is F1, so F1=0)

∑Momentum = 0 = - F2 - F3 + F4

0 = (-4 kg) (9.81 m/s2) (0.25m) - (6kg) (9.81 m/s2) (0.5-0.2m) + F4 (0.5-0.1m)

F4 = 68.67 N (right knife-edge)