Doubt in mass suspended by spring

[manimaran1605]

Consider a mass M, suspended by a weightless spring (horizontally or vertically). If I apply a force on the mass M, it executes simple harmonic motion. My question is for small displacement (that is pushed or pulled) according to Hooke's law F=-kx, it is equal to force we applied right?
F=Ma=-kx, Can i apply Newton's 3rd law here? if yes explain? (But According to Newton's third law, action and reaction happens in different bodies right?)

 

[Adithyan]

Consider a mass M, suspended by a weightless spring (horizontally or vertically). If I apply a force on the mass M, it executes simple harmonic motion. My question is for small displacement (that is pushed or pulled) according to Hooke's law F=-kx, it is equal to force we applied right?

What is equal to the force?

F=Ma=-kx, Can i apply Newton's 3rd law here? if yes explain? (But According to Newton's third law, action and reaction happens in different bodies right?)

Newton's 3rd law is applicable in every inertial frame.
If you are speaking of the reaction acting on you after you apply the force on the block, then the force acting on you will be equal to that of kx provided you exert the force slowly, i.e, without generating a kinetic energy in the block.