◄UNIT 5:FOSSIL, NON FOSSIL FUEL AND POWER PRODUCTIONUNIT 7: ATOMIC MODELS AND PHOTOELECTRIC EFFECT►

UNIT 6:MOTION IN ORBITS

  • UNIT 6:MOTION IN ORBITS

    People have always enjoyed viewing stars and planets on clear, dark nights. 
    It is not only the beauty and variety of objects in the sky that is so fascinating, 
    but also the search for answers to questions related to the patterns and 
    motions of those objects.
    Until the late 1700s, Jupiter and Saturn were the only outer planets identified 
    in our solar system because they were visible to the naked eye. Combined 
    with the inner planets the solar system was believed to consist of the Sun 
    and six planets, as well as other smaller bodies such as moons. Some of the 
    earliest investigations in physical science started with questions that people 
    asked about the night sky.
    i) Based on the scenario above and the observation from the picture. 
    Briefly summarize what is illustrated in the picture.
    ii) What is the name of belt separating the largest and smallest planets?
    iii) Explain why you think the moon doesn’t fall on the earth.
    iv) Why don’t we fly off into space rather than remaining on the Earth’s 
    surface? Explain your idea.

    v) Explain why planets move across the sky.

    Introduction 
    A natural phenomenon by which all things with mass or energy includingsatellites, 
    planets,stars,galaxies, and even light, are brought toward (or gravitate toward) 
    one another is referred to as gravity or gravitation. On Earth, gravity gives 
    weight to all physical objects around it.
    Gravity is very important to our everyday lives. Without Earth’s gravity we 
    would fly right off it. If you kicked a ball, it would fly off forever. While it might 
    be fun to try for a few minutes, we certainly can’t live without gravity. Gravity 
    also is important on a larger scale. 
    It is the Sun’s gravity that keeps the Earth in orbit around the Sun. Life on Earth 
    needs the Sun’s light and warmth to survive. Gravity helps the Earth to stay at 

    just the right distance from the Sun, so it’s not too hot or too cold.

    a) Discuss the interactions between two stones.
    b) Can the two stones attract one another? Explain your reasoning.
    c) Make a general conclusion about small bodies close to one another
    d) Imagine whether we have two bodies which are massive (too big), 
    explain the difference in interactions of massive bodies and small 

    bodies.Give any examples

    From the time of Aristotle, the circular motions of heavenly bodies were 
    regarded as natural. The ancients believed that the stars, planets, and Moon 
    moved in divine circles, free from any impressed forces. Newton, however, 
    recognized that a force must be acting on the Planets; otherwise, their paths 
    would be straight lines. 
    And whereas others of his time, influenced by Aristotle, said that any such 
    force would be directed along the planets’ motion, Newton reasoned it must 
    be perpendicular to their motion, directed toward the center of their curved 
    paths- toward the sun. This was the force of gravity, the same force that pulls 
    apples off trees.
    The Newton’s law of universal gravitation states that “Every particle in the 
    Universe attracts every other particle with a force that is directly proportional 
    to the product of their masses and inversely proportional to the square of the 
    distance between them”.






    Properties of Gravitational Force
    - It is always attractive in nature while electric and magnetic force can be 
    attractive or repulsive.
    - It is independent of the medium between the particles while electric 
    and magnetic forces depend on the nature of the medium between the 

    particles.

    - It holds well over a wide range of distances. It is found true for 
    interplanetary to inter-atomic distances.
    - It is a central force which means it acts along the line joining the centers 
    of two interacting bodies. 
    - It is a two-body interaction, where gravitational force between two 
    particles is independent of the presence or absence of other particles; so, 
    the principle of superposition is valid, and on the contrary, nuclear force 
    is a many-body interaction. 
    - It is the weakest force in nature. 
    - It is a conservative force,where work done by it is path independent or 
    work done in moving a particle round a closed path under the action of 
    gravitational force is zero. 
    - It is an action reaction pair,where the force with which one body (say, 
    earth) attracts the second body (say, moon) is equal to the force with 
    which moon attracts the earth. This is in accordance with Newton’s third 

    law of motion.

     

    Critically observe the picture above and answer the questions that follow.
    a) Describe the motion of bodies (planets) as indicated in the picture.
    b) Do the different planets pass through the same path? Explain to support your decision.
    c) With clear observations, which body is the largest. Explain to support your selection.
    d) Basing on the knowledge from Newton’s law of gravitation, is there 
    any force of attraction between the body stated in above and other 
    bodies? Explain your reasoning.
    e) If yes, how does the force affect the motion of the bodies as they are 
    in their paths?


    The Sun is not at the center of the ellipse, but is instead at one focus (generally 
    there is nothing at the other focus of the ellipse). 
    The planet then follows the ellipse in its orbit, which means that the Earth-Sun 
    distance is constantly changing as the planet goes around its orbit. 
    For purpose of illustration we have shown the orbit as rather eccentric; 

    remember that the actual orbits are much less eccentric than this. 

    6.2.2. Kepler’s Second Law
    It states that the line joining the planet to the Sun sweeps out equal areas in 

    equal times as the planet travels around the ellipse.

    Kepler’s second law is illustrated in the preceding figure. The line joining the 
    Sun and planet sweeps out equal areas in equal times, so the planet moves 
    faster when it is nearer the Sun. Thus, a planet executes elliptical motion with 
    constantly changing angular speed as it moves about its orbit. 
    What happen is best understood in terms of energy. As the planet moves away 
    from the Sun (or the satellite from Earth), it loses energy by overcoming the 
    pull of gravity, and it slows down, like a stone thrown upwards. And like the 

    stone, it regains its energy as it comes back

    The point of nearest approach of the planet to the Sun is termed perihelion; 
    the point of greatest separation is termed aphelion. Hence, by Kepler’s second 
    law, the planet moves fastest when it is near perihelion and slowest when it is 

    near aphelion. 

    6.2.3. Kepler’s Third Law
    It states that the ratio of the squares of the revolutionary periods for two 
    planets is equal to the ratio of the cubes of their semi major axes .Therefore, 

    the law is summarized in the expression below.

    - In this equation T represents the period of revolution for a planet and R 
    represents the length of its semi major axis. The subscripts “1” and “2” 
    distinguish quantities for planet 1 and 2 respectively. The periods for the 
    two planets are assumed to be in the same time units and the lengths of 
    the semi major axes for the two planets are assumed to be in the same 
    distance units. 
    - Kepler’s Third Law implies that the period for a planet to orbit the Sun 
    increases rapidly with the radius of its orbit. Thus, we find that Mercury, 
    the innermost planet, takes only 88 days to orbit the Sun but the outermost 
    planet (Pluto) requires 248 years to do the same. 
    - Kepler’s 3rd law applies only to objects orbiting the same attracting center. 
    Do not use to compare, say the Moon’s orbit around the Earth to the orbit 
    of Mars around the Sun because they depend on different attracting 

    centers.

    a) Verification of Kepler’s third law
    There is only one speed that a planet can have if the planet is to remain in an 
    orbit with a fixed radius. Since the gravitational force acting on the planet of 
    mass m in the radial direction, it alone provides the centripetal force. Therefore, 

    using Newton’s law of gravitation, we have:

    The mass m of planet does not appear in equation consequently, for a given 
    orbit, a planet with a large mass has exactly the same orbital speed as a planet 
    with a small mass. 
    The radius r of the orbit (distance from the center of planet to the center of the 
    sun) is in the denominator in equation. This means that the closer the planet is 
    to Sun, the smaller is the value for r and the greater the orbital speed must be.
    The period T of a planet is the time required for one orbital revolution. The 

    period is related to the speed of the motion by

    b) Verification of acceleration due to gravity at the surface of the earth
    The force of attraction exerted by the earth on a body is called gravitational pull 
    or gravity. We know that when force acts on a body, it produces acceleration. 
    Therefore, a body under the effect of gravitational pull must accelerate. The 
    acceleration produced in the motion of a body under the effect of gravity is 
    called acceleration due to gravity (g). 

    Consider a body of mass m lying on the surface of earth.

           

    1) Hold a balloon and fill it with air. Then let it go. In which direction 
    does the air come out of the balloon and in which direction does the 
    balloon get propelled?
    2) If you fill the balloon with water and then let the balloon go, does 
    the balloon’s direction change? Explain your answer.
    3) Based on the observations made on (a) and (b) above, analyze the 
    movement of the rocket shown in the figure above.
    4) Artificial satellites are machines launched in the atmosphere to 
    move around the Earth.
    (i) What is the instrument do you think is used to launch them in 
    the atmosphere?

    (ii)Discuss any roles of artificial satellites.

    1) Hold a balloon and fill it with air. Then let it go. In which direction 
    does the air come out of the balloon and in which direction does the 
    balloon get propelled?
    2) If you fill the balloon with water and then let the balloon go, does 
    the balloon’s direction change? Explain your answer.
    3) Based on the observations made on (a) and (b) above, analyze the 
    movement of the rocket shown in the figure above.
    4) Artificial satellites are machines launched in the atmosphere to 
    move around the Earth.
    (i) What is the instrument do you think is used to launch them in 
    the atmosphere?

    (ii)Discuss any roles of artificial satellites.

    6.3.1. Rockets
    A rocket is a missile, spacecraft, aircraft or other vehicle that obtains thrust 
    from a rocket engine. A rocket is a device that produces thrust by ejecting 
    stored matter (fuel). A rocket moves forward when gas expelled from the rear 
    of a rocket pushes it in the opposite direction. From Newton’s laws of motion, 

    for every action, there is an equal and opposite reaction. 

    Basic principle of Rocket propulsion
    Rocket propulsion is based on Newton’s laws of motion:
    - Momentum conservation law
    - Newton’s third law
    In a rocket, fuel is burned to make a hot gas and this hot gas is forced out of 
    narrow nozzles in the back of the rocket, propelling the rocket forward. 
    Factors Affecting a Rocket’s Acceleration
    - The greater the exhaust velocity of the gases relative to the rocket, the 
    greater the acceleration.
    - The faster the rocket burns its fuel, the greater its acceleration.
    - The smaller the rocket’s mass (all other factors being the same), the 
    greater the acceleration.
    Spacecraft Propulsion
    Spacecraft is a vehicle designed to operate, with or without a crew, in a controlled 
    flight pattern above Earth’s lower atmosphere.The spacecraft typically either 
    is placed into an orbit around Earth or, if given sufficient velocity to escape 
    Earth’s gravity, continues toward another destination in space. The spacecraft 
    itself often carries small rocket engines for maneuvering and orienting in space.
    Spacecraft Propulsion is characterized in general by its complete integration 
    within the spacecraft (e.g. satellites). Its function is to provide forces and 
    torques in (empty) space to:
    - Transfer the spacecraft: used for interplanetary travel 
    - Position the spacecraft: used for orbit control
    - Orient the spacecraft: used for altitude control
    The jet propulsion systems for launching rockets are also called primary 
    propulsion systems. Spacecrafts, e.g. satellites, are operated by secondary 
    propulsion systems.
    Characteristics of Spacecraft Propulsion Systems 
    In order to fulfill altitude and orbit operational requirements of spacecraft, 
    spacecraft propulsion systems are characterized by: 
    - Very high velocity increment capability ( km/s) 
    - Low thrust levels (1 mN to 500 N) with low acceleration levels 
    - Continuous operation mode for orbit control 
    - Pulsed operation mode for altitude control 
    - Predictable, accurate and repeatable performance (impulse bits)
    - Reliable, leak-free long time operation (storable propellants) 
    - Minimum and predictable thrust exhaust impingement effects 
    Classification of Propulsion Systems 
    Spacecraft propulsion can be classified according to the source of energy 
    utilized for the ejection of propellant: 
    - Chemical propulsion use heat energy produced by a chemical reaction 
    to generate gases at high temperature and pressure in a combustion 
    chamber. These hot gases are accelerated through a nozzle and ejected 
    from the system at a high exit velocity to produce thrust force.
    - Electric propulsion uses electric or electromagnetic energy to eject 
    matter at high velocity to produce thrust force. 
    - Nuclear propulsion uses energy from a nuclear reactor to heat gases 
    which are then accelerated through a nozzle and ejected from the system 
    at a high exit velocity to produce thrust force. 
    6.3.2. Satellites
    A satellite is an artificial or a natural body placed in orbit round the earth 
    or another planet in order to collect information or for communication. 
    Communication satellites are satellites that are used specifically to communicate. 
    The payload of communication satellite consists of huge collection of powerful 
    radio transmitters and or a big dish, to enable it to exchange information with 
    the ground. We use them to transmit TV signals, to transmit radio signals, and 
    in some cases, it transmits internet signals. 
    There is only one main force acting on a satellite when it is in orbit, and that 
    is the gravitational force exerted on the satellite by the Earth. This force is 
    constantly pulling the satellite towards the centre of the Earth.
    A satellite doesn’t fall straight down to the Earth because of its velocity. 
    Throughout a satellite’s orbit there is a perfect balance between the gravitational 
    force due to the Earth and the centripetal force necessary to maintain the orbit 
    of the satellite.
    Satellites are natural or artificial bodies describing orbit around a planet under 
    its gravitational attraction. Moon is a natural satellite while INSAT-1B is an 
    artificial satellite of the earth. Condition for establishment of artificial satellite 
    is that the centre of orbit of satellite must coincide with centre of earth or 
    satellite must move around great circle of earth.

    Orbital Velocity of Satellite







    6.3.3. Applications of satellites
    Satellites that are launched in to the orbit by using the rockets are called manmade satellites or artificial satellites. Artificial satellites revolve around the 
    earth because of the gravitational force of attraction between the earth and 
    satellites. Unlike the natural satellites (moon), artificial satellites are used in 

    various applications. The various applications of artificial satellites include:

    Weather forecasting, Navigation, Astronomy, Satellite phone, Satellite 
    television, Military satellite, Satellite internet and Satellite radio.
    1. Weather forecasting
    Weather forecasting is the prediction of the future of weather. The satellites that 
    are used to predict the future of weather are called weather satellites. Weather 
    satellites continuously monitor the climate and weather conditions of earth. 
    They use sensors called radiometers for measuring the heat energy released 
    from the earth surface. Weather satellites also predict the most dangerous 
    storms such as hurricanes.
    2. Navigation
    Generally, navigation refers to determining the geographical location of an 
    object. The satellites that are used to determine the geographic location of 
    aircrafts, ships, cars, trains, or any other object are called navigation satellites. 
    GPS (Global Positioning System) is an example of navigation system. It allows 
    the user to determine their exact location at anywhere in the world.
    3. Astronomy
    Astronomy is the study of celestial objects such as stars, planets, galaxies, 
    natural satellites, comets, etc. The satellites that are used to study or observe the 
    distant stars, galaxies, planets, etc. are called astronomical satellites. They are 
    mainly used to find the new stars, planets, and galaxies. Hubble space telescope 
    is an example of astronomical satellite. It captures the high-resolution images 
    of the distant stars, galaxies, planets etc.
    4. Satellite phone
    Satellite phone is a type of mobile phone that uses satellites instead of cell 
    towers for transmitting the signal or information over long distances.
    Mobile phones that use cell towers will work only within the coverage area of 
    a cell tower. If we go beyond the coverage area of a cell tower or if we reach the 
    remote areas, it becomes difficult to make a voice call or send text messages 
    with the mobile phones. Unlike the mobile mobiles, satellite phones have global 
    coverage. Satellites phones uses geostationary satellites and low earth orbit 
    (LEO) satellites for transmitting the information.
    When a person makes a call from the satellite phone, the signal is sent to the 
    satellite. The satellite will receives that signal, processes it, and redirects the 
    signal back to the earth via a gateway. The gateway then send the signal or
    call to the destination by using the regular cellular and landline networks. The 
    usage of satellite phones is illegal in some countries like Cuba, North Korea, 
    Burma, India, and Russia.
    5. Satellite television
    Satellite television or satellite TV is a wireless system that uses communication 
    satellites to deliver the television programs or television signals to the users or 
    viewers.
    TV or television mostly uses geostationary satellites because they look 
    stationary from the earth. Hence, the signal is easily transmitted. When the 
    television signal is send to the satellite, it receives the signal, amplifies it, and 
    retransmits it back to the earth. The first satellite television signal was send 
    from Europe to North America by using the Telstar satellite.
    6. Military satellite
    Military satellite is an artificial satellite used by the army for various purposes 
    such as spying on enemy countries, military communication, and navigation.
    Military satellites obtain the secret information from the enemy countries. 
    These satellites also detect the missiles launched by the other countries in the 
    space.
    Military satellites are used by armed forces to communicate with each other. 
    These satellites also used to determine the exact location of an object.
    7. Satellite internet
    Satellite internet is a wireless system that uses satellites to deliver the internet 
    signals to users. High-speed internet is the main advantage of satellite internet. 
    Satellite internet does not use cable systems, but instead it uses satellites to 
    transmit the information or signal.
    8. Satellite radio
    Satellite radio is a wireless transmission service that uses orbiting satellites 
    to deliver the information or radio signals to the consumers. It is primarily 
    used in the cars. When the ground station transmit signal to the satellite that 
    is revolving around the earth, the satellite receives the signal, amplifies it, and 

    redirects the signal back to the earth (radio receivers in the cars).


    ACROSS
    1. The only natural satellite of Earth. 
    5. An object in orbit around a planet. 
    6. The smallest planet and farthest from the Sun. 
    7. This planet probably got this name due to its red color and is 
    sometimes referred to as the Red Planet. 
    9. This planet’s blue color is the result of absorption of red light by 
    methane in the upper atmosphere.
    10. It is the brightest object in the sky except for the Sun and the 
    Moon.
    DOWN
    2. Named after the Roman god of the sea.
    3. The closest planet to the Sun and the eighth smallest.
    4. A large cloud of dust and gas which escapes from the nucleus of 
    an active comet.
    8. The largest object in the solar system.
    2) (i) Define astronomical satellite
     (ii) What does astronomical satellite used for? Give one example 
    of it.
    3) For a satellite to be in a circular orbit 780 km above the surface of 
    the earth, (a) what orbital speed must it be given, and (b) what is 

    the period of the orbit (in hours)?

             

                    

      


    ◄UNIT 5:FOSSIL, NON FOSSIL FUEL AND POWER PRODUCTIONUNIT 7: ATOMIC MODELS AND PHOTOELECTRIC EFFECT►