If we want to define escape and orbital velocities in simple words then we can say that orbital velocity is the speed required by an object to orbit around another object. Whereas, escape velocity is the minimum speed required by an object to break free from the gravitational pull of another mass. We will look at the concepts of escape and orbital velocities and their relationship in detail below.
Table of Contents
- What is Escape Velocity?
- Orbital Velocity
- Relationship Between Escape And Orbital Velocity
- Motion Of Satellites Around Earth
What is Escape Velocity?
We have studied in kinematics that the range of a projectile depends upon the initial velocity as
This minimum amount of velocity for which the particle escapes the gravitational sphere of influence of a planet is known as escape velocity (ve). When escape velocity is provided to a body it goes to infinity theoretically.
As the gravitational force is a conservative force, the law of conservation of energy holds good. Applying the law of conservation of energy for the particle which is provided with a required minimum velocity to go to infinity
At infinity, the particles experience no interaction so the final potential energy and we know from motion in 1D chapter the final velocity of a body becomes zero after reaching its maximum height so from this we can deduce the final kinetic energy of the particle.
From the above formula, it is clear that escape velocity does not depend upon the test mass (m).
If the source mass is earth then the escape velocity has a value of 11.2 km/s.
Escape Velocity – Video Lesson
The velocity with which the test mass orbits around a source mass is known as orbital velocity
If the test mass is at small distances from the source mass
Relationship Between Escape And Orbital Velocity
The relationship between escape velocity and orbital velocity can be mathematically represented as:
Meanwhile, if we divide Eq.(1) and Eq (2) we get,
It shows that escape velocity is
Certain conditions need to be taken into consideration. The main one is that the escape velocity should be square-root of 2 times larger than the orbital velocity to be free.
- When the velocities are the same, the object will be in constant orbit and the same elevation.
- If escape velocity is less than orbital then the orbit will diminish which will result in the object crashing.
- If it is more then the object will be free in the orbit and will likely float into space.
Motion Of Satellites Around Earth
Let us consider a satellite assumed to be revolving around the earth in a circular orbit of radius ‘r’ having the centre of the earth as its centre, let us see the kinematics of the satellite moving around the earth.
Here the test mass is a satellite and the source mass is earth. The velocity with which a satellite orbit around the earth is given by the orbital velocity,
Time Period Of A Satellite
The time taken by the satellite to complete one revolution around the earth is known as the time period of a satellite. The time period of a satellite is the ratio of the total distance travelled by the satellite around the earth to the orbital velocity.
which is Kepler’s 3rd law.
The constant of proportionality in the above equation depends only on the source mass but not on the test mass.
Kinetic Energy Of A Satellite
Kinetic energy is the energy possessed by the body in motion ( whether translational or rotational or a combination of both) in the case of the satellite orbiting around the earth. The satellite makes a pure rotational motion. In a pure rotation, the kinetic energy of the satellite is given by;
Where is the angular velocity of a satellite which is related to the time period of a satellite by a formula
From Kepler’s 3rd law we know the time period of a satellite, substituting it in the above formula we get
Kinetic energy can never be negative for any force.
Also Read: Kepler’s Laws
Potential Energy Of A Satellite
Potential energy is the energy possessed by the body in a particular position. Potential energy changes when the position of the body changes. To study potential energy we require two bodies, one is source mass(M) which provides the gravitational force and the other is test mass(m) which experiences the gravitational force by the source mass. In our case satellite is the test mass and earth is the source mass. The potential energy possessed by the satellite at a distance ‘r’ from the centre of the earth is given by;
Total Energy Of A Satellite
The total energy of the satellite is the sum of all energies possessed by the satellite in the orbit around the earth. As only the mechanical motion of the satellite is considered, it has only kinetic and potential energies.
Total energy of the satellite = kinetic energy of the satellite + potential energy of the satellite
From the above equation,
Total energy of the satellite = – (kinetic energy of the satellite)
Total energy of the satellite =
Virial theorem gives us the relationship between kinetic energy and potential energy. According to the virial theorem, if potential energy is proportional to nth power of position (r) then the average kinetic energy is equal to (n/2) times potential energy at that position.
Binding Energy Of A Satellite
Binding energy is the energy required to be given to the satellite to escape the gravitational pull of the earth. It is numerically equal to the negative of total energy possessed by the satellite.
Binding energy of a satellite =
The binding energy of a system of two bodies is the amount of minimum energy required to separate them by infinite distance or it is simply the amount of energy required to make the potential energy of the system equal to zero, which is numerically equal to the kinetic energy of the body under motion.
Angular Momentum Of A Satellite
When a satellite of mass ‘m’ is orbiting with an angular speed on the orbital path of radius ‘r’, its angular momentum is given by
We know that
From the above equation, it is clear that the angular momentum of a satellite depends on both the mass of a satellite and mass of earth. It also depends upon the radius of the orbit of the satellite.
Frequently Asked Questions on Escape and Orbital Velocity
What is escape velocity?
The minimum speed required to project a body vertically upwards from the surface of the earth so that it never returns to the surface of the earth is called escape velocity.
Define the orbital velocity of a satellite.
The velocity required to put a satellite into its orbit around the earth is called orbital velocity.
Hydrogen escapes faster from the earth than oxygen. Why?
The thermal speed of hydrogen is much larger than oxygen. Therefore, a large number of hydrogen molecules can acquire escape velocity than that of oxygen molecules. Hence hydrogen escapes faster from the earth than oxygen.
What is expression for escape velocity?
ve = √2gR
Here, R is the radius of the earth.