Question:

The distance between the sun and the earth be $r$, then the angular momentum of the earth around the sun is proportional to

  • $\sqrt{r}$
  • $r^{3/2}$
  • $r$
  • None of these
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The Correct Option is A

Solution and Explanation

By Kepler's third law of planetary motion,
we have, $T^{2} \propto a^{3}$
$\Rightarrow T \propto a^{3 / 2}$
Given a $=r$ and $\omega=\frac{2 \pi}{T}$
= angular speed $=\omega \propto r^{-3 / 2}$
Also angular momentum $L=m r^{2} \omega$
$\Rightarrow L \propto r^{2} \times r^{-3 / 2}$
$\Rightarrow L \propto r^{3 / 2}$
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Concepts Used:

Gravitation

In mechanics, the universal force of attraction acting between all matter is known as Gravity, also called gravitation, . It is the weakest known force in nature.

Newton’s Law of Gravitation

According to Newton’s law of gravitation, “Every particle in the universe attracts every other particle with a force whose magnitude is,

  • F ∝ (M1M2) . . . . (1)
  • (F ∝ 1/r2) . . . . (2)

On combining equations (1) and (2) we get,

F ∝ M1M2/r2

F = G × [M1M2]/r2 . . . . (7)

Or, f(r) = GM1M2/r2

The dimension formula of G is [M-1L3T-2].