# Thread: need help with equilibrium question!

1. ## need help with equilibrium question!

A satellite probe is put into an orbit around a distant planet such that at apoapsis its orbital radius is 4989 km and at periapsis its orbital radius is 2959 km. If it has a speed of 1.36 km/s at apoapsis what is its speed at periapsis?

2. Originally Posted by jddery
A satellite probe is put into an orbit around a distant planet such that at apoapsis its orbital radius is 4989 km and at periapsis its orbital radius is 2959 km. If it has a speed of 1.36 km/s at apoapsis what is its speed at periapsis?

HI

In an orbital system , the gravitational force=centripetal force from the energy principles again .

$\displaystyle \frac{GMm}{r^2}=\frac{mv^2}{r}$ ,

where G is the gravitational constant , M is the mass of planet , m : mass of satelite , r : orbitral radius

$\displaystyle v^2=\frac{GM}{r}$

Since GM are both constants , $\displaystyle v^2\propto \frac{1}{r}$

$\displaystyle v\propto \frac{1}{\sqrt{r}}$

At apoapsis , r=4989 , v=1.36

At periapsis , r=2959 , v=??

Now substitute appropriately ,

$\displaystyle 1.36 \propto \frac{1}{\sqrt{4989}}$ ----- 1

$\displaystyle v\propto \frac{1}{\sqrt{2959}}$ ---- 2

Divide them to get v which is approximately 1.77 km/s

3. Originally Posted by jddery
A satellite probe is put into an orbit around a distant planet such that at apoapsis its orbital radius is 4989 km and at periapsis its orbital radius is 2959 km. If it has a speed of 1.36 km/s at apoapsis what is its speed at periapsis?

$\displaystyle mv_ar_a = mv_br_b$
$\displaystyle v_a = \frac{v_b r_b}{r_a}$