Mechanics: Elastic Springs and Simple Harmonic Motion

I've seen a number of questions on this topic which confuse me. We often consider an elastic spring on a smooth horizontal surface with the particle at one end, and given that it moves with SHM and having calculated the frequency and/or amplitude, we have to calculate the velocity of the particle when the spring is slack.
In this scenario, only the tension in the spring need be considered, so the maximum velocity is when the tension is nil, i.e. when at natural length. I approached these questions by use of the standard SHM formula vmax=2pifA, but I always get a completely different answer to that if I consider elastic potential energy becoming kinetic energy, which gives the right answer. Why cannot I simply use the equation? :confused:

Reply 1

ghostwalker

17

Original post by Nuclear Ghost

I've seen a number of questions on this topic which confuse me. We often consider an elastic spring on a smooth horizontal surface with the particle at one end, and given that it moves with SHM and having calculated the frequency and/or amplitude, we have to calculate the velocity of the particle when the spring is slack.
In this scenario, only the tension in the spring need be considered, so the maximum velocity is when the tension is nil, i.e. when at natural length. I approached these questions by use of the standard SHM formula vmax=2pifA, but I always get a completely different answer to that if I consider elastic potential energy becoming kinetic energy, which gives the right answer. Why cannot I simply use the equation? :confused:

It would help if you posted an actual question, with your working, so we can see what's going on. On the face of it, I can't see any reason why you should get a different answer.

Reply 2

TeeEm

19

Original post by Nuclear Ghost

I've seen a number of questions on this topic which confuse me. We often consider an elastic spring on a smooth horizontal surface with the particle at one end, and given that it moves with SHM and having calculated the frequency and/or amplitude, we have to calculate the velocity of the particle when the spring is slack.
In this scenario, only the tension in the spring need be considered, so the maximum velocity is when the tension is nil, i.e. when at natural length. I approached these questions by use of the standard SHM formula vmax=2pifA, but I always get a completely different answer to that if I consider elastic potential energy becoming kinetic energy, which gives the right answer. Why cannot I simply use the equation? :confused:

Both methods should give the same answer unless you have external non conservative forces such as ground friction. Weight and spring tension are conservative( i.e can be derived from potentials) so they conserve energy.

As ghostwalker suggested post a problem with clear workings then someone could help you

Reply 3

Nuclear Ghost

OP

14

Original post by TeeEm

Both methods should give the same answer unless you have external non conservative forces such as ground friction. Weight and spring tension are conservative( i.e can be derived from potentials) so they conserve energy.

As ghostwalker suggested post a problem with clear workings then someone could help you