[填空题]
A small box is held in pla2 xz6jbf kehs .)t6yj6ce against a rough wall by so)bb sjx7ijx:9zz ez64z 3krm2meone pushing on it with 9mz6 kxj3b4e:s)bi jz2 z7 zxra force directed upward at 28$^{\circ}$ above the horizontal. The coefficients of static and kinetic friction between the box and wall are 0.40 and 0.30 , respectively. The box slides down unless the applied force has magnitude 13
N. What is the mass of the box?
