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Homework Help: Central Force problems using radial motion equation

  1. Oct 6, 2015 #1
    1. The problem statement, all variables and given/known data
    a satellite is in a circular orbit a distance $h$ above the surface of the earth with speed $v_0$, booster rockets are fired which double the speed of the satellite without changing the direction. Find the subsequent orbit.
    2. Relevant equations
    3. The attempt at a solution
    Before the rocket boost, If we use the radial motion equation we can find the total energy of the system, that is given by:

    $$E = \frac{1}{2}\dot{r}^2 + V + L^2/2r^2 = \frac{v_0^2}{2} - \frac{\gamma}{R_e + h}$$

    Where I have assumed the presence of an attractive inverse square law.
    and $L = rv_0$.

    How can I continue from here? surely when the velocity increases, it isn't going to continue in a circular orbit.

    After the boost, we have $$\frac{1}{2}\dot{r}^2 + V + L^2/2r^2 = \frac{1}{2}\dot{r}^2 - \frac{L^2}{2r^2} - \frac{\gamma}{r} = E $$

    we can write

    $$\frac{1}{2}4v_0^2 - \frac{L^2}{2r^2} - \frac{\gamma}{r} = E $$

    then equating

    I obtain

    $$\frac{L^2}{2r^2} + \frac{\gamma}{r} = \frac{\gamma}{R_e + h} + \frac{3}{2v_0^2}$$

    I don't see how the solutions to the above equation describe the orbit
  2. jcsd
  3. Oct 6, 2015 #2


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    Won't it escape the Earth? Since before the boost we have ##\frac{v^2}{r}=\frac{GM}{r^2}## where ##M## is the Earth's mass, we have ##v=\sqrt{\frac{GM}{r}}##, the gravitational potential energy is ##-\frac{GMm}{r}=-mv^2## and the total potential energy is ##-\frac{1}{2}mv^2##. If the speed is doubled, the KE goes to ##2mv^2## and the total energy goes to ##mv^2##, which is positive, implying escape on a hyperbolic trajectory (ignoring the fact that it will be captured by the Sun).

    So I guess by orbit, they mean the hyperbolic path it will follow. The equations for that path should be readily obtainable from a page on orbital trajectories such as this.
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