3.4.1.5
Newton's first law: no resultant force means no change in motion
Newton's Laws & Momentum — AQA A-Level Physics
Key Definition
Newton's first law — An object will remain at rest or move with constant velocity unless acted on by a resultant force.
- A resultant force is needed to change an object's speed, direction, or both.
- If forces are balanced ($resultant = 0)$, the object is either at rest or moving at constant velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹..
- This law defines what happens in the absence of a net force.
- Balanced forces: forces $left = forces right$, forces $up = forces down$.
Examiner Tips and Tricks
- A common misconception is that a moving object must have a force acting on it.
- Newton's first lawAn object remains at rest or moves with constant velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹. unless acted on by a resultant force. says the opposite: constant velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹. requires zero resultant force.