Răspuns:
m=800 g=0,8 kg Ec_A=mv,/2<=>90=0,8v,/ 2<=>90=0,4vo²=>vo²=225%=>Vo=v225=15 m/s AEc_AB=L_G+L_Ff+L_N AEc_AB=Ec_B-Ec_A=-Ec_A L_G=-mgh;L_N=0 ;L_Ff=-Ff*AB=-µABmgcosa =-µmghctga Ec_A=mgh+µmghctga=>h=Ec_A/ [mg(1+µctga)]=90/[0,8*10(1+0,5*ctg30°)]=90/ [8(1+0,5*1,73)]=90/15=6 m sina=h/BC=>BC3H/sina3D6/1/2=12 m BC=12 m L_Ff=-µmghctga=-0,5*0,8*10*6*1,73=-41,52 J Em=Ec+Ep Ec_B=0 Em=Ep=mgh=0,8*10*6=48 J