Fields Tutorial 3 - Satellite Motion

Motion of Masses in Gravitational Fields

Newtons Laws of Gravitation can be used to explain the motion of planets and stars.  Much of modern space exploration uses the three hundred-year-old model.  Orbiting satellites are NOT doing gravity defiance acts; instead they are actually falling in a curved path towards the Earth all the time.  However they have a sufficient forwards velocity to miss the Earth all the time.  Since the gravity field is radial, the force acts at 90o to the direction of travel all the time.  Therefore the path is circular.  If we stopped gravity, the satellite would fly off tangentially into space in a straight line.  If we stopped the satellite, it would fall straight back to Earth.

For a satellite to be in a particular orbit, a particular velocity is required or a given distance.  Some satellites are placed so that they go in an easterly direction, completing one orbit each day.  They remain above one given point on the Earths surface, so are called geostationary.  This kind of satellite orbit is used in telecommunications.  Other satellites move in a polar orbit so that they can perform sweeps of the surface.  Spy satellites use a polar orbit.

We can derive an expression for the geostationary satellite.  We know that:


So we can write:

The m terms cancel out:

The term v2/r = acceleration:

We also know that:

We can therefore write:

The minus signs cancel.  We can simplify the expression to:

However, frequency, f, is not a very useful term; the period T is more helpful:

Since T = 1/f, we can write:

Our final rearrangement gives:

This relationship is called Kepler's Third Law.

Worked Example

A communications satellite is to be placed in a circular geostationary orbit.  What must its height and speed be?

                                 This question seems to be remarkably lacking in information, but there is an answer.


             Use Newtons Laws of Gravitation to solve this.  There is a single force acting on the satellite, gravitational attraction, so the satellite is acceleration all the time towards the centre of the Earth.


             We need the satellite to be travelling at a sufficient forwards velocity so that it completes ONE orbit every 24 hours.  We need to work out the angular velocity before we can work out the linear speed.


w = 2pf.  We need to work out f

                 f = ____1_____ = 1.16 10-5

24 60 60


w = 2 p 1.16 10-5 Hz = 7.27 10-5 rad/s.


             Now we need to consider the centripetal acceleration:


a = w2r.

 We also know that the acceleration is also given by g = -GM/r2


w2r = GM/r2 [We will ignore the minus sign.]


r3 = -GM = 6.67 10-11 N m2 kg-1 5.98 1024 kg = 7.55 1022 m3

(7.27 10-5 Hz)2


r = 3 7.55 1022 m3 = 4.24 107 m


               But we need to take away the radius of the Earth to get the height:


Height = 4.24 107 m - 6.37 106 m = 3.60 107 m = 36000 km


               Now we can work out the forward velocity:


v = wr = 7.27 10-5 rad/s 4.24 107 m = 3100 m/s.



Question 1

The planet Mars has a diameter of 6800 km.  A satellite is in orbit 5000 km above the planet's surface travelling at a speed of 7100 m/s. 

(a)                How long does it take to orbit? 

(b)               What is the centripetal acceleration at this speed?

(c)                What is the acceleration due to gravity at this distance?

(d)                Will it remain in that orbit?  Mass of Mars = 6.42 1023 kg





Graphs from Kepler III

If we plot a graph of T2 against r3, we get a straight line.


If we observe the orbital period of four different satellites and know the radius of their orbits, we see that the graph is a straight line.  The data fit the straight line neatly.  Orbits are very slightly elliptical, but to a first approximation, they are circular.


We can also plot the graph of T2/3 against rT2/3 is the cube root of the square of T.


Escape Velocity of a Rocket

This refers to the minimum velocity that is needed to move a rocket, mass m, from the Earths surface to infinity.  We need to consider the work that needs to be done:

This work has to be done against gravity, and the energy for that work is provided by the fuel.

The object gains kinetic energy which is equal to the potential energy.

Therefore we can write:

Notice that the minus sign has gone.  This is because we are doing a job of work against the gravity field.  The masses cancel out, so we can say:

This then gives:

We know that gravitational field strength, g, is given by:

Which we can rearrange to:


The r2 term cancels out to r.  We take the square root to give our final expression:

Question 2

What is the escape velocity of a rocket from the Earth? (g = 9.8 N kg-1; r = 6.37 106 m)


Question 3

A black hole is an object that has such a high mass that light, travelling at 3 108 m s-1, cannot escape.

(a) What is the escape velocity of such a black hole?

(b) Calculate the mass of such a black hole, assuming that it has the same radius of the Earth.

(c) Compare the mass of the black hole with the mass of the Sun.