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Circular Motion (Edexcel GCSE Physics)
Revision Note
Circular Motion
Higher Tier Only
- 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:
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
Examiner Tip
You may be asked to explain why motion in a circle involves constant speed but changing velocity, so remember to mention that velocity is a vector quantity, so both magnitude and direction are important.Even though the magnitude (speed) doesn’t change, its direction does - so the velocity itself is changing.
Centripetal Force
Higher Tier Only
- An object moving in a circle is not in equilibrium as it has a resultant force acting upon it
- This is known as the centripetal force and is what keeps the object moving in a circle
- The centripetal force (F) is defined as:
The resultant perpendicular force towards the centre of the circle required to keep a body in uniform circular motion
- The centripetal force is shown by the arrow labelled F in the diagram below:
Centripetal force is always perpendicular to the direction of travel and is directed towards the centre of the circle
- Note: centripetal force and centripetal acceleration act in the same direction
- This is due to Newton’s Second Law
- The centripetal force is not a separate force of its own
- It can be any type of force, depending on the situation, which keeps an object moving in a circular path
Examples of Centripetal Force Table
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