# Spring and mass

1. Mar 5, 2006

### atomant

I got a question. If a mass hangs motionless from a spring, what is the force exerted on the mass by the spring in terms of the spring constant, k?
So I was thinking when a load of mass m is used on the spring it will stretch by a distance x, and as the extention is directly propotional to the weight the spring should obey Hooke's law. Thus mg=-kx. However the queston asks for the force exerted by the mass and I am not too sure how that is obtained. There is obvioisly no SHM, so I can;t use T=2pi sqroot m/k. any ideas as to how this is solved?

2. Mar 5, 2006

### andrewchang

$\Sigma F = 0$

the force exerted by the mass is its weight.

3. Mar 5, 2006

### topsquark

Specifically, the mass is in equilibrium. Thus the force the spring is exerting on the mass is equal to its weight. The force the mass is exerting on the spring is equal to the force the spring is exerting on the mass, by Newton's Third Law.

-Dan

4. Mar 5, 2006

### atomant

Of course!!. I have been going around in circles Thanks!