Thread: Kinetic, Gravitational Potential and Elastic Potential Energy

I have a question involving the extension of an elastic string that has me a little stumped.

The set up of the question is as follows:

Particle P of mass 0.5kg is attached to one end of a light, elastic string that has a natural length of 2m, and a modulus of elasticity of 20N.

The other end of the string is attached to a ceiling, and P is held 1.5 metres directly below the point of attachment, before being released.

What is the length of the string when P reaches its lowest point?

To me, the question relies on calculating the Kinetic Energy of P as the string becomes taught (i.e. how much Kinetic Energy is gained in the 0.5m before the tension in the string begins to play a role), and then determining the amount of potential energy stored in the string when the Kinetic Energy becomes zero (at which point I expect the energy in the string to be equal to the loss in gravitational potential energy since the release of P). Hence:











From here I just need to solve for y (i.e. the extension of the string) and add on the requisite 2m, but I'm not coming up with the right answer (which should be 3.34m). I seem to be missing something in the set up of the problem, can anyone see what it is? Thanks very much.

In your equations, y is the length of the string, not the extension (otherwise you should not subtract 2m).

Using your ansatz: 5N/m * (y-2m)^2 = 1/2kg*9.81m/s^2 * (y-1.5m)
(y/m-2)^2 = 0.981*(y/m-1.5)
This has the solutions y=1.64 (unphysical) and y=3.35, which is the right answer.

Originally Posted by mfb

In your equations, y is the length of the string, not the extension (otherwise you should not subtract 2m).

Not quite sure how I managed to make that error and not notice it! Thanks a lot.