Tom is travelling on a train which is moving at a constant speed of 15 m s- on a horizontal track. Tom has placed his mobile phone on a rough horizontal table. The coefficient of friction
between the phone and the table is 0.2. The train moves round a bend of constant radius. The phone does not slide as the train travels round the bend. Model the phone as a particle
moving round part of a circle, with centre O and radius r metres. Find the least possible value of r