- #1

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i dont understand why d(mv)/dt = m(dv/dt) + v(dm/dt)

can somone help ?

TNX !

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- Thread starter sedaw
- Start date

- #1

- 62

- 0

i dont understand why d(mv)/dt = m(dv/dt) + v(dm/dt)

can somone help ?

TNX !

- #2

ehild

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How do you calculate the derivative of a product?

ehild

ehild

- #3

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This is simply obtained by using the product rule.

d**p**/dt = d/dt (m**v**) = m(d**v**/dt) + **v**(dm/dt) (using **product rule**).

d

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- #4

- 182

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I thought that Newton's second law was true only for systems with constant mass.

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- #5

- 97

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[tex] F=m \frac{dv}{dt} [/tex]

it is valid inly for constant mass system since you assume that the mass is constant :P.

But assuming, that mass is dependant on velocity, which is true (according to SR) you get what sedaw wrote.

Thus, the best, most general form of Newton's 2nd law is

[tex] F=\frac{dp}{dt} [/tex]

as it doesn't imply anything being constant ;P

- #6

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[tex] F=m \frac{dv}{dt} [/tex]

it is valid inly for constant mass system since you assume that the mass is constant :P.

But assuming, that mass is dependant on velocity, which is true (according to SR) you get what sedaw wrote.

Thus, the best, most general form of Newton's 2nd law is

[tex] F=\frac{dp}{dt} [/tex]

as it doesn't imply anything being constant ;P

I know that [tex]\bold{F}=\frac{d\bold{p}}{dt}[/tex] is Newton's second law, but I read that this was true only with constant masses. Also, I don't know much about special relativity (I just finished my freshman year) but I thought it proved that Galilean transformation was flawed. Since Newton's laws are based on Galilean relativity, I don't think SR can show that N2 holds even for variable masses. Can it?

- #7

diazona

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If I remember correctly, in special relativity, Newton's second law (in the above form) is used to

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