4. A box \(A\) of mass 0.8 kg rests on a rough horizontal table and is attached to one end of a light inextensible string. The string passes over a smooth pulley fixed at the edge of the table. The other end of the string is attached to a sphere \(B\) of mass 1.2 kg , which hangs freely below the pulley. The magnitude of the frictional force between \(A\) and the table is \(F N\). The system is released from rest with the string taut. After release, \(B\) descends a distance of 0.9 m in 0.8 s . Modelling \(A\) and \(B\) as particles, calculate
a the acceleration of \(B\),
b the tension in the string,
c the value of \(F\).
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