(a) Apply the Momentum Principle. Define the system as the HST. Identify the forces on the HST and sketch a freebody diagram for the HST. The only force on the HST is the gravitational force by Earth that acts radially, toward Earth as shown below.
Figure: The orbit of HST.
According to the momentum principle, the net force perpendicular to the path of an object is
where R is the radius of its path. Now, sum the forces acting in the radial direction on the object. In this case, there is only one force, the gravitational force by Earth. Substitute this for the net force.
Now, solve for the speed of the orbit. Note that the mass of the telescope cancels out of the equation.
Substitute the mass of the telescope and the radius of its orbit. Note that we are given its altitude, measured from Earth's surface. Its orbital radius is measured from the center of Earth which is the radius of Earth plus its altitude. Be sure to convert km to m.
The orbital speed is
(b) The orbital period is found using the definition of average speed for an object moving in circular motion.
Convert the period to hours.
(c) To find the work done on the HST to put it into orbit, apply the energy principle. Define the system as the HST and Earth. Treat both Earth and the HST as point particles, meaning that they only have rest energy and kinetic energy. And of course, there is energy due to their gravitational interaction, which is called gravitational potential energy. The energy principle states that
Substitute the energy of the system which is the sum of the particle energies and their interaction energy.
Write the particle energy as the sum of its rest energy and kinetic energy. Note that rest energy cancels out of both sides of the equation, if we assume that the mass of the HST and Earth haven't changed during the process.
Solve for the work done on the system by the surroundings.
Substitute expressions for kinetic energy and gravitational potential energy.
Now, we need to determine values for initial and final speed and initial and final distance from Earth. Initially, the HST is at rest on Earth, but Earth is rotating with a period of 24 hours. The average radius of Earth and HST's initial distance from the center of Earth is . Thus, the initial speed of the HST is
The final speed and final distance of the HST in orbit was computed in part (a) as and . Substitute all known values into our expression for work.
As expected, the work done on the system is positive. Because the HST in orbit is further away from Earth, the system has a greater potential energy. Also, in orbit it is traveling faster (7900 m/s) than when on the launch pad on Earth (460 m/s). Thus, the final energy of the system is greater than the initial energy of the system. As a result, the work done on the system should be positive, according to the energy principle. Our result at least has the correct sign.
