Gravity and Gravitation Central Force A central force between two particles is one which is directed along the line joining the two particles and whose magnitude is a function of the distance between them. If one particle is fixed in its position, the central force acting on the other is $\vec{F}$, then the torque acting on it is: $$ \vec{\tau} = \vec{r} \times \vec{F} = 0 $$ Since, $$ \vec{\tau} = \frac{d\vec{J}}{dt} $$ $$ 0 = \frac{d\vec{J}}{dt} $$ $$ \Rightarrow \vec{J} = \text{constant} $$ Again, we know, $$ \vec{J} = \vec{r} \times \vec{p} $$ $$ \vec{r} \cdot \vec{J} = \vec{r} \cdot (\vec{r} \times \vec{p}) = (\vec{r} \cdot \vec{r})\, \vec{p} = 0 $$ This shows that $\vec{r}$ and $\vec{J}$ are perpendicular to each other, and the motion of the particle is confined to a plane. Central force can be represented as: $$ \vec{F} = \frac{C}{r^2}\,\hat{r} $$ For gravitational force, $$ C = Gm_1 m_2, \qquad \text{then} \qquad \vec{F} = \frac{Gm_1 m_2}{r^2}\,\hat{r} $$ For...