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Question:
It is easy to explain to students what is happening with respect to Newton's Third Law when you stand on a surface and you are not moving. You press down, the surface presses up, and the net force is zero. What happens when you step onto a surface with a rotten plank and you fall through? Clearly the forces are not balanced. If the action and reaction forces are equal and opposite, does this mean that, regardless of your weight, that only the portion of your weight that was sufficient to break the plank is the action force, and, therefore, the reaction from the plank is just not sufficient to stop your descent?



Replies:
The action force is constant; it is the pull due to gravity on your body. The plank provided a short term and insufficient reaction force. Unfortunately, as the plank broke, the reaction force from the plank disappeared and you are now falling with the force due gravity accelerating you until you hit the ground. On the way down, the reaction force is you pulling Earth up. Nothing has changed in the action force, but reaction force is different.

Bob Avakian

Oklahoma State University Institute of Technology.



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