Circular Motion (CIE IGCSE Physics)

Extended tier only

• Velocity is a vector quantity with both magnitude and direction
• Therefore, the velocity of an object is its speed in a given direction

• When an object travels in circular motion, its direction is always changing
  • Therefore, the velocity of an object in circular motion is always changing, even if its speed is constant
• Circular motion is a type of acceleration since acceleration is a change in velocity

Circular motion of the International Space Station

The International Space Station’s velocity is always changing - it whizzes around the Earth at a constant speed of about 7660 m/s but is always changing direction

 

• When a force acts perpendicularly to an object’s direction of travel, the force will cause that object to change direction

 

When the two cars collide, the first car changes its direction in the direction of the force

 

• If the force continues to act at 90 degrees to the motion, the object will keep changing its direction (whilst remaining at a constant speed) and travel in a circle
• This is what happens when a planet orbits a star, or when a satellite orbits a planet

Circular motion of the Moon around the Earth

The Moon is pulled towards the Earth (at 90 degrees to its direction of travel). This causes it to travel in a circular path

• Therefore, for an object in circular motion, the force is always directed toward the centre of the circle

 

• The force needed to make something follow a circular path depends on a number of factors:
  • The mass of the object
    
    • A greater mass requires a greater force when the speed and radius are constant
  • The speed of the object
    
    • A faster-moving object requires a greater force when the mass and radius are constant
  • The radius of the circle
    
    • A smaller radius requires a greater force to keep the speed and radius constant