It takes a spring constant of 50 N/m to hold a spring streched to a distance of 40 cm. what is the elastic potential energy of the spring at this position ?
If (and only if) by 'streched to a distance of 40cm' you mean that it's 40 cm from irs rest position, then the elastic potential energy can be written [tex]E=\frac12kl^2=\frac12\cdot50\cdot(.4)^2=\boxed{4J}[/tex]
