Can someone please explain why marks were taken off in this limit question • Physics Forums

FrancisG · 2018-02-15T01:02:55+00:00

My teacher says I have to solve all the limits at once, but I don't understand why it is mathematically incorrect to solve one limit before the other. The test is here:

Thread 'How does time derivative commute from one variable to another?'

I'm reviewing Meirovitch's "Methods of Analytical Dynamics," and I don't understand the commutation of the derivative from r to dr: $$ \mathbf{F} \cdot d\mathbf{r} = m \ddot{\mathbf{r}} \cdot d\mathbf{r} = m\mathbf{\dot{r}} \cdot d\mathbf{\dot{r}} $$

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MHB Can someone please explain why marks were taken off in this limit question

Thread 'How does time derivative commute from one variable to another?'

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