Khi treo quả cân 300g
\(F_{đh}=k.\Delta l\Leftrightarrow3=k.\left(31-l_0\right)\)
Khi treo thêm quả cân 200g
\(F_{đh}=k.\Delta l\Leftrightarrow2=k.\left(33-31\right)\)
\(\Leftrightarrow k=1\left(N/cm\right)\)
\(\Rightarrow3=1.\left(31-l_0\right)\Leftrightarrow l_0=28\left(cm\right)\)