Question

in lewins lecture about newtons laws of motion he demonstrates a string holding a box and another string hanging feely from the bottom .he pulls the bottom one and says the top one should break by newtons laws but bottom one broke.why and how?

Answer 1

If you pull the string quickly, the inertia of the box resists the change in motion and the tension in the lower string is greater than the upper string. If you pull slowly enough, the box begins to move downward (albeit very slowly and nearly imperceptibly) and the inertia acts maintains this motion and the tension in the upper string is greater.