Circular Motion (Cambridge (CIE) O Level Physics): Revision Note

Circular Motion

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

• When an object travels along a circular path, its velocity is always changing

  • The speed of the object moving in a circle might be constant - that is, it is travelling the same distance every second

  • However, the direction of travel is always changing as the object moves along the circular path

• This means that an object moving in circular motion travels at a constant speed but has a changing velocity

• The image below shows an example of a famous object that moves in a circular path with a constant speed but changing direction:

International Space Station Orbiting Earth

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 at 90 degrees to an object’s direction of travel, the force will cause that object to change direction

 An Impact Force Causing a Change in the Motion of a Car

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 satellite orbits a planet)

The Moon Orbiting 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

• 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