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Deducing Kepler's second law from newton's laws?

  1. Aug 25, 2015 #1
    I've searched a little bit and found that I can derive kepler's third law from newton's law of gravitation. That's okay. But I want to deduce kepler's second law too: "An imaginary line joining a planet and the sun sweeps out an equal area of space in equal amounts of time".
    I know it's possible to do that using angular momentum, and from there proving that angular momentum in this case is constant. But how would I do that?
  2. jcsd
  3. Aug 25, 2015 #2
  4. Aug 26, 2015 #3
    Keep the sun at the origin. At any time t let ##\vec { r(t)}## be the position vector of a planet. At the moment t+dt the position vector will be ##\vec {r (t)}+\vec{dr (t)}##. So within the interval of time dt the planet sweeps out an area ## \vec{da}= \frac {1}{2} \vec{r (t)}\times\vec{dr (t)} \Rightarrow \vec{\frac{da}{dt}}= \frac {1}{2} \vec{r (t)}\times\vec{\frac{dr}{dt}} ## Now you know that Newton's gravitational force is central in nature or mathematically ## \frac {d^2\vec{r}}{dt^2} = f (r)\vec{r} \Rightarrow \vec{r}\times \frac {d^2\vec{r}}{dt^2} = \vec{0}\Rightarrow \frac {d}{dt}(\vec{r (t)}\times\vec{\frac{dr}{dt}})=\vec{0}\Rightarrow \frac{d}{dt}(\vec{\frac{da}{dt}})=\vec{0} ## So, ##\vec{\frac{da}{dt}}## is a constant vector. This is Kepler's second law.
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