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I was doing a physics problem out of my textbook, which could paraphrased as follows:
There’s a frame suspended at rest from a coiled spring. A mass of flexbile putty is dropped onto the frame, causing the frame and putty to move downwards together. What is the maximum distance the frame goes downward?
The problem could clearly modeled as a collision followed by an energy conservation problem.
However, after thinking about it, I couldn’t come up with a justification why momentum should be conserved in this inelastic collision, considering that the net external forces on the frame-putty system are not zero (putty experiences gravitational force, frame experiences gravitational and elastic, right?).
After reviewing the solution, they give no justification why momentum should be conserved in this case, but they utilize an inelastic collision with momentum conservation.
Any thoughts on what I’m missing here?