• When planets move around the Sun, or a moon moves around a planet, they orbit in?circular motion
  • This means that in one orbit, a planet travels a distance equal to the circumference of a circle (2πr) describing that orbit

   

• Using the relationship between speed, distance and time, the orbital speed of an object can be defined by the equation:

• Where:
  • v?= orbital speed in metres per second (m/s)
  • r?= orbital radius in metres (m)
  • T?= time period?in seconds (s)

   

• This time period is defined as:

The time taken for an object to complete one orbit

• The orbital radius?r?is always taken from the?centre?of the object being orbited to the object orbiting

Orbital radius and orbital speed of a planet moving around a Sun

Step 1: List the known quantities

• • Radius of the Earth = 6400 km
  • Distance of the telescope above the Earth's surface = 560 km
  • Time period,?T?= 96 minutes

   

Step 2: Write the relevant equation

Step 3: Calculate the orbital radius,?r

• • The orbital radius is the distance from the?centre?of the Earth to the telescope

   

r?= Radius of the Earth + Distance of the telescope above the Earth's surface

r?= 6400 + 560 = 6960 km

Step 4: Convert any units

• • The time period needs to be in seconds

   

1 minute = 60 seconds

96 minutes = 60 × 96 = 5760 s

• • The radius needs to be in metres

   

1 km = 1000 m

6960 km = 6 960 000 m

Step 5: Substitute values into the orbital speed equation