# 3.1.3 Newton's Laws of Motion

### Newton's Three Laws of Motion

• Newton’s First Law: A body will remain at rest or move with constant velocity unless acted on by a resultant force Newton’s First Law on a car at constant velocity

• Newton’s Second Law: A resultant force acting on a body will cause a change in momentum in the direction of the force. The rate of change in momentum is proportional to the magnitude of the force
• This can also be written as F = ma Using Newton’s Second law to find the mass of an accelerating skateboarder

• Newton’s Third Law: If body A exerts a force on body B, then body B will exert a force on body A of equal magnitude but in the opposite direction
• Newton’s Third Law force pairs must act on different objects
• Newton’s Third Law force pairs must also be of the same type e.g. gravitational or frictional Newton’s Third Law to describe the motion of an untied balloon

#### Exam Tip

You may have heard Newton’s Third Law as: ‘For every action is an equal and opposite reaction’. However, try and avoid using this definition since it is unclear on what the forces are acting on and can be misleading. Newton’s Third Law force pairs are only those that act on different objects ### Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.
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