Physics HL · Chapter 2: Forces and Newton's Laws
2.3 Newton's First, Second, and Third Laws
Use Newton's laws as a unified framework for inertia, acceleration, and interaction pairs.
Estimated time: 31 minutes
First Law and Inertial Thinking
Newton's first law states that zero net force corresponds to constant velocity. This includes the special case of rest. The deeper message is conceptual: motion does not need a sustaining force, only a changing force. That insight separates modern mechanics from older push-to-keep-moving intuition.
Inertia captures resistance to changes in motion state. Large mass means a stronger acceleration-changing force is needed for the same acceleration outcome. This prepares the second law, where force, mass, and acceleration are connected quantitatively.
Second Law as the Quantitative Engine
Newton's second law turns diagrams into predictions: net force equals mass times acceleration. The acceleration direction is always the net-force direction. If competing forces nearly cancel, acceleration is small. If one direction dominates, acceleration aligns with that direction.
Most exam questions reduce to this pattern: identify forces, sum by axis, solve for acceleration or unknown force. Keep units coherent and avoid mixing scalar magnitudes with signed vector components. Correct sign discipline is often the difference between physically sensible and impossible answers.
A dependable five-step loop mirrors the classic exam workflow: choose a positive direction, draw the free-body diagram, write the net-force expression from that diagram, set it equal to ma, then solve. Most sign errors are caught by forcing yourself to write the net-force line explicitly before substituting numbers.
Third Law Pairs Without Confusion
Newton's third law says interaction forces come in equal-magnitude opposite-direction pairs acting on different bodies. This is why action-reaction forces never cancel each other on a single free-body diagram: each member of the pair belongs to a different object's diagram.
A useful check is to name both bodies in every force label. For example, 'force of table on block' pairs with 'force of block on table'. This naming prevents accidental cancellation of quantities that should remain in separate equations.
Worked Logic: Two-Body Systems and Internal Tension
For two connected masses, you can solve in two complementary ways. Method A: write F = ma separately for each body, then solve simultaneously for acceleration and tension. Method B: treat both masses as one system to find acceleration quickly, because internal tension cancels inside the combined system. Then return to one body to recover tension.
These relations follow directly from applying Newton's second law to each mass with consistent sign choices.
Apply component-by-component after a complete free-body diagram.