A Box (M = 20 Kg) Is Sitting on a Horizontal

Multiple Choice

A box (m = 20 kg) is sitting on a horizontal surface. It is connected to a massless hook by a light string passing over a massless pulley wheel. The coefficients of friction between the box and the surface are 0.50 (static) and 0.30 (kinetic) . On top of m is a second box M of mass 20 kg. The coefficients of friction between the boxes m and M are 0.80 (static) and 0.60 (kinetic) . How much weight needs to be added to the hook until the box just begins to move.

A) 10 kg
B) 98 kg
C) 20 kg
D) 16 kg
E) 200 kg