Khi vat qua vi tri can bang thi co nang ton tai duoi dang dong nang nen giu co dinh chinh giua se khong lam thay doi co nang toan phan
Mat khac giu co dinh chinh giua thi $k'=2k$
$<=>W'=W<=>\frac{1}{2}kA^2=\frac{1}{2}k'A'^2<=>\frac{A'}{A}=\sqrt{\frac{k}{k'}}=\sqrt{1}{\sqrt{2}}$
$<=>A'=\frac{A}{\sqrt{2}}$