TBR physics sec 4

[exquisitemelody]

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Had a question about one of the passages, Physics section 4 passage 1 (a child's momentum) problem 4.

It's saying that momentum isn't conserved because the log that the child/sled hits exerts a force on the child/sled that is considered an external force. How is that external? Why aren't they an action-reaction pair?

 

[ravupadh]

It is a little confusing, but I think they wanted you to treat the sled and child as a an action-reaction pair throughout the entire passage. Therefore initial and final energies are in relation to the sled/child system only (i.e. before the child hit the log and after she got stuck to it), and any loss of momentum/energy because of "external forces" (i.e. the log) will explain why the system hasn't conserved momentum or energy.