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
