ΔEc_AB=L_G+L_N
ΔEc=Ec_B-Ec_A=mv₂²/2-mv₁²/2
L_G=mgh ; L_N=0
mv₂²/2-mv₁²/2=mgh |:m/2=>v₂²-v₁²=2gh<=>v₂²=2*10*2+3²=40+9=49=>v₂=√49=7 m/s
ΔEc_BC=L_G+L_N+L_Ff
L_G=0 ; L_N=0 ; L_Ff=-μmgd
Ec_C=0
ΔEc_BC=Ec_C-Ec_B=-mv₂²/2
-mv₂²/2=-μmgd |:(-m)/2=>v₂²=2μgd<=>7²=0,8*10*d<=>49=8d=>d=49/8=6,12 m