Calculating Orbital Speeds (CIE IGCSE Physics)

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• 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 (the shape of the orbit)
  • This is equal to 2πr where r is the radius a circle 

• The relationship between speed, distance and time is:

• the average orbital speed of an object can be defined by the equation:

• Where:
  • v = orbital speed in metres per second (m/s)
  • r = average radius of the orbit in metres (m)
  • T = orbital period in seconds (s)

• This orbital period (or 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, R = 6400 km
  • Distance of the telescope above the Earth's surface, h = 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

Step 4: Convert any units

• • The time period needs to be in seconds

• • The radius needs to be in metres

Step 5: Substitute values into the orbital speed equation