Newton's First Law
Forces in Action - OCR A-Level Physics
Key Definition
Newton's First Law
An object remains at rest or continues to move at constant velocity unless acted upon by a resultant external force.
An object remains at rest or continues to move at constant velocity unless acted upon by a resultant external force.
- In a free body diagramA diagram showing all the forces acting on a single object, drawn as arrows from the centre of the object., draw the object as a dot or simple shape and show ALL forces acting ON that object as labelled arrows
- Arrow length should be proportional to force magnitude
- Do NOT include forces that the object exerts on other objects (those are third-law pairs)
- If the resultant force is zero, the object is in equilibriumAn object is in equilibriumAn object is in equilibrium when the resultant force on it is zero. The object is either stationary or moving at constant velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹.. when the resultant force on it is zero. The object is either stationary or moving at constant velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹.. -- either stationary or moving at constant velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹.
- An object moving at constant velocity has zero resultant force, not zero force
Common Mistake
MEDIUM
Students often: You might say an object at rest has 'no forces acting on it'.
Instead: An object at rest has no RESULTANT force. Individual forces still act (e.g. weight and normal contact force) but they are balanced.
Instead: An object at rest has no RESULTANT force. Individual forces still act (e.g. weight and normal contact force) but they are balanced.