# Homework Help: Tension Force and SHM

1. Jun 18, 2011

### Peter G.

Hi,

I am learning about SHM and in order to explain why the acceleration is proportional to the displacement but in opposite directions the book used as an example a particle with mass m, attached to a spring.

He defines the displacement to the right as being positive and to the left as being negative.

When the particle is displaced to the right, x, the extension, will be positive. The tension in the spring is towards the left. But why? Is it a characteristic of the spring to always have its force trying to restore the spring to its equilibrium position?

Thanks,
Peter G.

2. Jun 18, 2011

### Staff: Mentor

Absolutely. Ideal springs obey Hooke's law.

And whenever the restoring force is proportional to the displacement from equilibrium, the resulting motion will be simple harmonic motion.

3. Jun 18, 2011

### Encephalon

Yes. This is how Hooke's law is typically expressed $$F=-kx$$
Where x is the displacement from the equilibrium position and k is the spring constant. There are a number of different and non-intuitive ways to come to this conclusion (you will cover springs in differential equations and a course on thermal/statistical physics), but the physically intuitive understanding is that a spring will apply a force to a mass at any point other than it's equilibrium position. If it is moved away from that position, the spring then applies a force to move it in, and, in an undamped case, will continue applying force until the mass is past the equilibrium position in the opposite direction, to which the spring will then a apply a force in reverse. This leads to oscillation, and will be infinitely periodic in the undamped and frictionless case.

4. Jun 18, 2011

### Peter G.

Ok, thanks guys!