11.A stone is accelerated upwards by a cord whose breaking strength is three times the weight of the stone. The maximum acceleration with which the stone can be moved up without breaking the cord is

12.n small balls each of mass m impinge elastically each second on a surface with velocity u. The force experienced by the surface will be

13.A block has weight W and it is held against a vertical wall by applying a horizontal force F. The minimum value of F is

14.A boy stand on a weighing machine inside a lift . When the lift is going down with ecceleration g/4, the machine shows a reading 30 kg. When the lift goes upwards at acceleratin g/4, the reading would be

15.A uniform rope of mass M, length L is fixed at its upper end vertically from a rigid support. Then the tension in the rope at the distance l from the rigid support is