Spring Constant

Experiments using “optical tweezers” measure the elasticity of individual DNA molecules. For small enough changes in length, the elasticity has the same form as that of a spring. A DNA molecule is anchored at one end, then a force of 1.5 nN (1.5 x 10^-9 N) pulls on the other end, causing the molecule to stretch by 5.0 nm  (5.0 x 10^-9 m) .

What is the spring constant of that DNA molecule?

We have to derive “k” from Hooke’s Law

F=kx

k = F/x

= 1.5 x 10^-9 N/5.0 x 10^-9 m

= 0.3 N/m