A charged particle enters a magnetic field with its velocity at right angles to the field. Its motion will be such that

its kinetic energy varies, while its momentum remains unchanged
its momentum varies, while its kinetic energy remains unchanged
neither its momentum nor its kinetic energy stays constant
both its momentum and its kinetic energy stay constant

Correct answer: its momentum varies, while its kinetic energy remains unchanged

Solution

The magnetic force is perpendicular to velocity, so it does no work: kinetic energy (and speed) stay constant. Momentum is a vector whose direction continuously changes, so momentum varies.

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