1. A scientist has set up a device to measure the frictional resistance force between a block B and a rough horizontal tabletop.  Block B has a mass of 4.5 kg, and it is connected by a light inextensible string to a metal sphere A of mass 3.5 kg.  The string passes over a smooth light fixed pulley at the edge of the table so that A is hanging vertically downwards as shown in the diagram below:

The string between B and the table is horizontal, and the frictional resistance between B and the table is modelled as a constant force of magnitude F_f  N.

The system is released from rest with the string taut and with sphere A exactly 1 metre above the ground.

Given that sphere A strikes the ground 0.8 seconds after it is released, determine the value of F_f.