# 1.5.4 Circular Motion

Extended Only

### 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

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 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)
• The speed of the object (a faster-moving object requires a greater force)
• The radius of the circle (a smaller radius requires a greater force)

### Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
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