Newton's Law
Learning Articles
Summary
Common Points of Confusion
| Misconception | Clarification |
|---|---|
| “No motion means no forces.” | False. Forces can be balanced (net force = 0), resulting in zero acceleration—but the object may be at rest or moving at constant velocity. |
| “Action–reaction forces cancel.” | Only if both act on the same object—but they never do. They act on different objects, so they don’t cancel in a single FBD. |
| “Normal force always equals $mg$.” | Not true. $\vec{N}$ is a constraint force that adjusts to prevent interpenetration. On an incline, $N = mg\cos\theta$; in an elevator, it changes with acceleration. |
| “Tension is the same everywhere.” | Only for massless, inextensible strings with no friction at pulleys. Otherwise, tension can vary. |
| “Centrifugal force is real.” | In inertial frames, no—it’s a fictitious force used only in rotating (non-inertial) frames to preserve $\vec{F}=m\vec{a}$. |
| “Mass and weight are the same.” | Mass ($m$) is scalar and invariant. Weight is the gravitational force: $\vec{W} = m\vec{g}$. |
Summary
Newton’s laws form a cause–effect framework linking forces to motion—but they must be applied:
- In an inertial reference frame,
- To a clearly defined system,
- With attention to the vector nature of force and acceleration,
- And with careful distinction between internal and external forces.