## anonymous one year ago Help

1. anonymous

Its a derivation from momentum changes in a system of variable mass..my Question is that why a sign of velocity of big mass relative to small mass is changed in 2nd step..

2. anonymous

@mathmate

3. anonymous

@shamim

4. anonymous

@IrishBoy123

5. IrishBoy123

can hardly read it but they also switched U and V around to get relative velocity so i presume that it assists presentation

6. anonymous

7. mathmate

Yeah, it's very hard to read. Can you write it out using the drawing tool (the "draw" button below)?

8. anonymous

No i am unable to use drawing tool because i am using tablet ..

9. IrishBoy123

clearly you are starting with $$\large \vec F = \frac{d}{dt}\vec p = \frac{d}{dt} (m \vec v)$$ and running with that gives $$\large m \frac{d \vec v} {dt} +\frac{dm}{dt} (\vec v - \vec u)$$ and then the switch to $$\large m \frac{d \vec v} {dt} -\frac{dm}{dt} (\vec u - \vec v)$$ where $$(\vec u - \vec v)\ = \vec v_{rel}$$ all looks very OM except we do not actually know what you are modelling! eg what is $$\vec u$$ ?

10. IrishBoy123

"very OM" = very OK

11. anonymous

U is velocity of big mass where v is the velocity of ejected mass

12. anonymous

I think u r right tht signs doesnt mean.. this iw due to relativity..

13. Michele_Laino

since the subsequent identity, has been used: $\Large {\mathbf{v}} - {\mathbf{u}} = - \left( {{\mathbf{u}} - {\mathbf{v}}} \right)$

14. anonymous

Okay thanks to all for helping me.. :)