Newton's First Law
Newton's Laws & Momentum - OCR A-Level Physics
Key Definition
Newton's First Law
The velocityThe rate of change of displacement. A vector quantity. Measured in $\text{m s}^{-1}$. of an object will only change if a net (resultant) force is acting on it. Equivalently: an object stays at rest or moves at constant velocity unless a resultant force acts.
The velocityThe rate of change of displacement. A vector quantity. Measured in $\text{m s}^{-1}$. of an object will only change if a net (resultant) force is acting on it. Equivalently: an object stays at rest or moves at constant velocity unless a resultant force acts.
Case 1 : Resultant force is zero
The object does not accelerateThe rate of change of velocity. A vector quantity. Measured in $\text{m s}^{-2}$.. It is either at rest, or it continues at constant velocity in a straight line. Change in momentum is zero.
$$\Sigma F = 0 \quad \Rightarrow \quad \Delta p = 0$$
Case 2 : Resultant force is non-zero
The object accelerates. Acceleration counts as starting to move, stopping moving, speeding up, slowing down, or changing direction. The first law alone tells us this happens; the second law tells us how much.
- Stationary book on a table: weight (down) and normal contact (up) cancel, so $\Sigma F = 0$ and the book stays at rest.
- Skydiver at terminal velocity: drag (up) equals weight (down), so $\Sigma F = 0$ and velocity is constant even though motion continues.
- Car moving at constant 30 $\text{m s}^{-1}$ on a flat road: driving force equals resistive forces, so $\Sigma F = 0$. No acceleration.
- The first law defines inertiaThe tendency of an object to resist changes in its state of motion. Greater mass means greater inertia.: the tendency of an object to resist changes in its motion. Mass is the measure of inertia.
Common Mistake
MEDIUM
Wrong: Writing that the first law says "an object needs a force to keep moving". This is the Aristotelian view, not Newton's.
Right: A moving object keeps moving at constant velocity in a straight line when the resultant force is zero. A force is only needed to change the velocity, not to maintain it.
Right: A moving object keeps moving at constant velocity in a straight line when the resultant force is zero. A force is only needed to change the velocity, not to maintain it.
Examiner Tips and Tricks
- "Constant velocity" means constant speed AND constant direction. Circular motion at constant speed still involves acceleration because direction changes.
- When asked to "state Newton's first law" for full marks, include both halves: behaviour when $\Sigma F = 0$ AND that any change of velocity requires a resultant force.
Free-body diagram: book at rest on table with weight (down) and normal contact (up) labelled and clearly equal in length.