# Question #bc4fb

Jan 13, 2018

Normally you would measure the stiffness (k) from doing a Hooke’s law investigation alongside the main practical.

#### Explanation:

If you know the stiffness or spring constant, the A’s work done = energy transfer, $E = F \times s$ where s is the displacement and $F = k \times e$ where e is the extension of the spring/elastic band.

Thus we can say that $E = k \times {s}^{2}$

It can be done from your data, because you know the coefficient of friction, you can find the frictional force from ${F}_{f} = \mu \times R$ and then, assuming all the work done is against friction (ignoring air resistance, hysteresis of the eleastic band etc.) the energy stored = energy released = ${F}_{f} \times s$ where s is again the displacement (distance travelled.)

You could presumably use the mass and ave. velocity to find the kinetic energy, make similar assumptions regarding energy transfers being all against friction and compare your two results to get some measure of uncertainty, but I think this method will not be as precise.