8.11 - Orbits

• An orbit is the curved path of one celestial object or spacecraft, around another celestial object.
• Which of Newton's three laws is this:

  "An object travelling at a certain velocity will continue to travel at that velocity unless acted upon by a resultant force."

  First law
• The diagram above shows the moon orbiting the earth.

  Which arrow, A or B, shows the direction of the instantaneous velocity?

  A shows the moons instantaneous velocity as that is the direction the moon will be travelling in. B shows the moons weight, i.e. the force of gravity pulling it towards the Earth. 

  
• The diagram above shows the moon orbiting the earth.

  Which arrow, A or B, shows the direction of the force of gravity?

  B

  A shows the moons instantaneous velocity as that is the direction the moon will be travelling in. B shows the moons weight, i.e. the force of gravity pulling it towards the Earth.

  
• Velocity is a vector quantity which means it has both magnitude and direction.

  During orbit, one of these quantities is constantly changing, which means the velocity is also constantly changing.

  Which quantity is constantly changing?

  Direction
• Which statement about objects in orbit is true?

  • The speed is constant but the velocity is changing
  • Velocity is a vector quantity and so has both a magnitude and direction. As an object orbits, its direction will change, and therefore its velocity will also change. 
• Which of these correctly explains whether an orbiting object is accelerating, and why?

  The object is accelerating because the velocity is changing
• If an object moves closer to the object it's orbiting, it will experience a greater force of gravitational attraction. 

  The magnitude of the velocity must increase
• What provides the force needed to keep a satellite in its orbit?

  • Gravity
  • If it wasn't for gravity, the satellite would just continue travelling straight, and travel away from the object it was orbiting. 
• What is an orbit?

  The curved path of one celestial object around another
• Why do objects orbit rather than fly off in a straight line?

  Due to the balance of gravitational force and velocity
• What does Newton's first law state about an object's motion?

  An object will continue at constant velocity unless acted upon
• If the moon travels at $1023 \text{ m/s}$, what is this speed called?


  Instantaneous velocity
• How does the Earth's mass affect the moon's orbit?

  It exerts a strong gravitational pull on the moon
• What happens to the moon's direction due to Earth's gravitational force?

  The moon's direction is constantly changing
• Why is the moon's velocity constantly changing?

  Because its direction is always changing
• What does the equation for acceleration relate to in terms of velocity?

  Any change in velocity indicates acceleration
• What must happen when the moon changes direction in its orbit?

  It must be accelerating
• What are the two key factors that result in a steady orbit?

  • Instantaneous velocity of the orbiting object
  • Gravitational pull of the larger object
• What happens if the moon's orbit is pulled in closer to the Earth?

  The gravitational force will increase significantly
• What must the moon do to maintain a stable orbit if it gets closer to Earth?

  It must travel much faster
• What is the relationship between the size of an object's orbit and its speed?

  Smaller orbits require faster travel speeds
• What is the takeaway regarding orbit size and speed?

  • Smaller orbits require faster speeds
  • Otherwise, objects will be pulled in