Let's calculate the difference in force, $\Delta F$, experienced by the rocks. Because $\Delta r$ is very small compared to $r$, $$\Delta F = F{\text{out}} - F{\text{in}} \approx \frac{dF}{dr}\Delta r = -\frac{2GMm}{r^3} \Delta r$$ What's the significance of that dF/dr Δr? or in a general case y = dy/dx Δx What's with this Δx/dx ? Please tell me which topic it is so I can just study it . I'm searching book after books. Sorry for uploading image. Downvotes are welcomed but just tell me from where to study it and then you can close my question.

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