Which of the following properties of a proton moving freely in a magnetic field can change?
(A) Mass
(B) Speed
(C) Velocity
(D) Both speed and velocity
Generated by claude-sonnet-4-6 · 2026-06-26 01:10 · grounding rag
Model Answer
(C) Velocity
A magnetic force is always perpendicular to the velocity, so it changes the direction of motion but does no work, leaving speed (and hence kinetic energy and mass) unchanged. Since direction changes, velocity changes.
Explanation
- Speed is a scalar (magnitude only) — it stays constant in a magnetic field.
- Velocity is a vector — its direction changes continuously (circular motion), so velocity changes.
- Mass is invariant.
- Option D ("both speed and velocity") is wrong because speed does NOT change.
- Examiners expect you to identify velocity as the correct answer and briefly justify using the perpendicular nature of magnetic force.