Newton's three laws explain how forces affect motion: an object maintains constant velocity unless a resultant force acts, a resultant force changes momentum, and forces always occur as equal and opposite interactions. These laws are part of IB Physics A.2 Forces and momentum at both SL and HL.
The Three Laws
| Law | Statement and meaning | Key equation or example |
|---|---|---|
| First law | In an inertial reference frame, an object remains at rest or moves with constant velocity unless acted on by a non-zero resultant external force. This property is associated with inertia. | If , then , so velocity is constant. |
| Second law | The resultant force equals the rate of change of momentum. For constant mass, this becomes force equals mass multiplied by acceleration. | , or . |
| Third law | When body A exerts a force on body B, body B simultaneously exerts an equal-magnitude, opposite-direction force of the same type on body A. |
Applying the Laws
Suppose a trolley experiences a driving force of and friction of . The resultant force is
Newton's second law gives
The trolley pushes backward on the ground while the ground pushes forward on the trolley with an equal force. These form a third-law pair because they act on different bodies.
A common misconception is that constant velocity requires a forward force. It does not: constant velocity requires zero resultant force. Another misconception is that third-law forces cancel; they do not cancel because they act on different objects.
Exam Technique
For an IB explain question, connect force to the resulting change in motion. Draw a free-body diagram containing only forces acting on the chosen body, calculate the resultant force, and then apply Newton's second law. Never place both forces of a third-law pair on the same free-body diagram.
