Maybe too soon to talk about having everything something to zero oh, how we do that with dealing with different frames of reference, Etc. Perhaps we should start with the idea of calculating the existence of a net force and say that net force causes the acceleration.
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Net Force diagrams should sum to zero.
This is like an accountant telling you that your books should balance, or some to zero.
Assume you tell an accountant, “hey wait a minute, my expensed were 800 and my revenue was 1000, that is a profit of 200, that’s not zero!
The accountant stares at you and then says the $200 profit is another expense, and 1000 minus 800 minus 200 equals zero. The accountants will not stop, until the final answet is zero.
We do the same thing with a net force diagram. After we have everything added up, may the result
Rob Sterling 