Circular motion and orbital speed: Space physics: Physics: GCSE (9:1)

• Velocity

  The speed of an object in a given direction
• Acceleration


  The rate of change of speed or velocity
• Resultant force


  A single force that replaces all the forces acting on an object and causes an object to accelerate
• Circular motion


  When an object follows a circular path, usually at a constant speed

  
• Velocity (object in circular motion)


  The velocity of an object travelling in a circle is changing (even when its speed is constant) because its direction is changing

  
• Acceleration (object in circular motion)


  An object travelling in a circle is accelerating because its direction (and therefore its velocity) is changing

  
• Centripetal force

  The resultant force that causes the acceleration of an object travelling in a circle

  
• Direction of centripetal force

  The centripetal force always acts towards the centre of a circle

  
• Gravitational attraction


  Provides the centripetal force that keeps satellites in orbit around planets and planets in orbit around stars

  
• How the force of gravity between two objects varies with distance

  The closer the two objects are, the stronger force of gravity between them

  
• Effect of the force of on velocity

  The greater the force of gravity between two objects, the greater acceleration and therefore the velocity so the object will move faster
• Effect of orbit size on the speed of an satellite

  Objects in small orbits travel faster than objects in large orbits
• Polar orbit


  Satellites in a polar orbit travel over the Earth’s poles

  
• Height above the Earth for a satellite in a polar orbit

  The satellites travel as low as 200 km above sea level

  
• Speed of a satellite in a polar orbit

  The satellites have a small orbit so travel very fast, at nearly 8,000 m/s

  
• Geostationary orbit


  Satellites in a geostationary orbit take 24 hours to orbit the Earth so appear to remain above the same point on the Earth's surface

  
• Height above the Earth for a satellite in a geostationary orbit

  The satellites travel around 36,000 km above sea level

  
• Speed of a satellite in a geostationary orbit

  The satellites have a large orbit so travel slower than a satellite in a polar orbit, at around 3,000 m/s

  
• Relationship between orbital speed of a satellite and its height above the Earth's surface

  The further from the Earth's surface a satellite is, the slower it travels
• Relationship between orbital speed of planet and its distance from the Sun

  The further from the Sun a planet is, the slower it travels