x Turn on thread page Beta
 You are Here: Home >< Maths

# Simple Harmonic Motion watch

1. I really don't understand Simple Harmonic Motion when there are two elastic strings. I've looked at it, and I really don't understand...

For example, this question;

Particle mass 0.2kg, connected by two light elastic strings, each natural length 0.5m, modulus of elasticity 5N, and connected to points A and B on a smooth horizontal table. Midpoint of AB is O, and AB is 1m. The particle is displaced 0.3m towards B, to Point C and released from rest. Show that after t seconds, the displacement from O being x metres, that d^2x/dt^2 = -100x.

I have no idea where to begin, and so as much help as possible would be greatly appreciated.
2. The force acting on the particle is only due to the string in tension. This force, from basic elasticity stuff:

Also,

Using this, acceleration can be calculated in terms of displacement.

EDIT: That said, I don't seem to be able to get the answer. Hmm...

EDIT again: I seem to be able to get the answer if I treat the strings and springs, that can be compressed, however, by doubling the force up.
3. (Original post by sean712)
I really don't understand Simple Harmonic Motion when there are two elastic strings. I've looked at it, and I really don't understand...

For example, this question;

Particle mass 0.2kg, connected by two light elastic strings, each natural length 0.5m, modulus of elasticity 5N, and connected to points A and B on a smooth horizontal table. Midpoint of AB is O, and AB is 1m. The particle is displaced 0.3m towards B, to Point C and released from rest. Show that after t seconds, the displacement from O being x metres, that d^2x/dt^2 = -100x.

I have no idea where to begin, and so as much help as possible would be greatly appreciated.
Are you sure they weren't springs?
4. Oh Yeah, sorry, they are springs :/

So how did you get the answer? I've tried it and keep getting confused and ended up with a=-30 and no where near the answer...

EDIT: I understand now! Thank you for your help

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: March 27, 2011
Today on TSR

### University rankings 2019

Cambridge at number one

Poll
Useful resources

### Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams