by action-reaction forces, if restoring force in a spring is equal to the force applied(in a vertical spring) how is that the mass is moving?(isnt it in equilibrium?

Question

anonymous · Answer

how is that it is oscillating?

anonymous · Answer

The forces are balanced if we stretch the spring and hold it there in equilibrium. Once the action force is removed, the spring force will act on the mass causing it to accelerate towards the equilibrium position. At the equilibrium position, the force of the spring is zero, but the velocity of the mass is at a maximum. Since the mass possess a velocity it has momentum, therefore it will continue past the equilibrium point. The spring force then acts to oppose this motion. Eventually, the mass comes to rest and the spring force then accelerates the mass back towards the equilibrium point. This pattern continues forever in the absence of external forces.

anonymous · Answer

what if i give only the weight mg(exerted on spring by weight) and no external force by me?

anonymous · Answer

The spring will stretch to a new equilibrium position. 

Let's say that a spring, right out of the box and unloaded, has a length $L$ and a spring constant $k$. If attach the spring at one end to the ceiling and at the other end we attach a mass, $m$. The spring will stretch to a new equilibrium length. This new length will be $L' = L + x$ where $x = {mg \over k}$. The spring-mass system will NOT oscillate in this case.

anonymous · Answer

is it beacuse of action-reaction forces?
also,
as u say it a necessary condition that the spring be compressed(for all cases) beyond its normal lenghth?(as the spring goes beyond mean position because of inertia?)

anonymous · Answer

Yes. The spring-mass system will not move because the spring force balances the gravitational force. 

Yes. In an oscillating system, the mass continues past the equilibrium position because of momentum (a function of inertia). Since Newton's Second Law says that$$F = {dp \over dt}$$The force must act over some finite amount of time to decrease the momentum to zero. During this time, the mass will have travelled some distance past the equilibrium point. The spring cannot simply stop the motion of the mass at the equilibrium point if it is in motion.

anonymous · Answer

I must go to class now. I will check back later to address any further doubts you may have. In the mean time, http://en.wikipedia.org/wiki/Simple_harmonic_motion and hyperphysics.phy-astr.gsu.edu/hbase/shm.html

mani_jha · Answer

Always remember that action and reaction are equal in maginitude, opposite in direction, but they act on different bodies. You apply a force on the spring to strectch it. The spring applies an equal force on your hand(you can even feel the pull it exerts on you). Since these forces act on different bodies, they dont necessarily balance each other.