1. Figure 1 shows a small block $B$ of mass 4.5 kg resting on a rough horizontal table. Block $B$ is attached to a small metal sphere $A$ of mass 3.5 kg by a light, inextensible string.

The string passes over a small, smooth, fixed pulley at the edge of the table so that the portion of the string between $B$ and the pulley is horizontal and the portion between the pulley and $A$ is vertical.

The resistance to motion between block $B$ and the table is modelled as a constant horizontal force of magnitude ${F}_{f}$.

Initially the string is taut and sphere $A$ hangs 1.0 m above the ground. The system is released from rest, and 0.80 s later sphere $A$ strikes the ground.

Determine the value of ${F}_{f}$.