4. A non-uniform plank \(A B\) of mass 20 kg and length 6 m is supported at both ends so that it is horizontal. When a woman of mass 60 kg stands on the plank at a distance of 2 m from \(B\), the magnitude of the reaction at \(A\) is \(35 g \mathrm {~N}\).

1. Suggest a suitable model for
   1. the plank,
   2. the woman.
2. Calculate the magnitude of the reaction at \(B\), giving your answer in terms of \(g\).
3. Explain briefly, in the context of the problem, the term 'non-uniform'.
4. Find the distance of the centre of mass of the plank from \(A\).