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M5 variable mass

In the edexcel textbook, when you use the change in momentum = impulse principle and there is an example of a rocket moving under gravity, it uses (m+§m)g as the weight but in the exam questions in mark schemes it uses mg as the weight so which one should we use?. I know it disappears later on in the 1st principles when you set the limits but just want to make sure on which one we should use in the exam.

Thanks
Original post by kennz
In the edexcel textbook, when you use the change in momentum = impulse principle and there is an example of a rocket moving under gravity, it uses (m+§m)g as the weight but in the exam questions in mark schemes it uses mg as the weight so which one should we use?. I know it disappears later on in the 1st principles when you set the limits but just want to make sure on which one we should use in the exam.

Thanks


For the impulse it doesn't matter which version you use for the weight. The extra term will vanish later anyway. For the change in momentum part, you must use m + dm for the final mass and m for the initial mass.
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kennz
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Original post by tiny hobbit
For the impulse it doesn't matter which version you use for the weight. The extra term will vanish later anyway. For the change in momentum part, you must use m + dm for the final mass and m for the initial mass.


That's what I thought, thank you!🙂

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