Hello! I was wondering whether sb could help me with this one. I don't seem to understand the solution to question 10. Here's the link to it http://ocw.mit.edu/courses/physics/8-01sc-physics-i-classical-mechanics-fall-2010/concept-of-force/MIT8_01SC_quiz07_sols.pdf . In the 2nd case, now that we have 2 bodies suspended from the spring, it's said that the spring stretches by half the amount as in the 1st case. I mean, we do have 2 bodies, each one stretching the spring by half the amount as in the 1st case, so doesn't that make it a whole?

Question

Hello! I was wondering whether sb could help me with this one. I don't seem to understand the solution to question 10. Here's the link to it http://ocw.mit.edu/courses/physics/8-01sc-physics-i-classical-mechanics-fall-2010/concept-of-force/MIT8_01SC_quiz07_sols.pdf .  In the 2nd case, now that we have 2 bodies suspended from the spring, it's said that the spring stretches by half the amount as in the 1st case. I mean, we do have 2 bodies, each one stretching the spring by half the amount as in the 1st case, so doesn't that make it a whole?

anonymous · Answer

Okay scratch that, refer to this site: http://www.physicsforums.com/archive/index.php/t-172192.html Forces acting on both ends of the spring do not add up because the force acting on the spring is equal to the tension of the whole rope. I'll refer to case A as an example. |dw:1345793628469:dw| Even in case A, the spring is influenced by two forces. The one above the spring is caused by the reaction force that came from pulling the spring. Refer to Newton's third law of motion. So in case B, this reaction force is replaced by the second mass on the opposite side, which causes the equilibrium. I'll illustrate again: |dw:1345793866289:dw| Hope this explanation helped. :)