JEE Main · 2004 · 24 Apr 2004 · Offline · Laws of Motion

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A particle is acted upon by a force of constant magnitude which is always perpendicular to the velocity of the particle, the motion of the particle takes place in a plane. It follows that

(A)its velocity is constant
(B)its acceleration is constant
(C)its kinetic energy is constant✓ Correct
(D)it moves in a straight line.

Answer: (C)

Solution

A force that always acts at right angles to the velocity does no work, since work needs a component of force along the displacement. With no work done, the work-energy theorem keeps the kinetic energy fixed; the force only swings the direction of the velocity, never its size, so the particle rounds a circular path at constant speed and its kinetic energy stays constant.

JEE Main · 2004 · 24 Apr 2004 · Offline · Single correct option

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