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 equilibriumA state in which the resultant force AND the resultant moment on an object are both zero., so it is either stationary or moving at constant velocityThe rate of change of displacement. A vector quantity, measured in $\text{m s}^{-1}$..
- A car cruising at $30 \text{ m s}^{-1}$ has zero resultant force: the driving force from the engine equals the total of drag and friction.
- Add an equation to the law: if $\sum F = 0$ then $a = 0$, and if $a = 0$ then $\sum F = 0$.
- An object moving at constant velocity has zero resultant force, not zero force
Common Mistake
MEDIUM
Students often: say an object at rest has 'no forces acting on it'.
Instead: An object at rest has no RESULTANT force. Individual forces (such as weight and normal contact force) still act but balance to zero.
Instead: An object at rest has no RESULTANT force. Individual forces (such as weight and normal contact force) still act but balance to zero.
$$\sum F = 0 \iff a = 0$$
Examiner Tips and Tricks
- If a question says "the object moves at constant velocity", that is your trigger to write $\sum F = 0$.
- State this explicitly in the answer; markers reward the link between constant velocity and zero resultant force.