JEE Main · 2019 · 11 Jan 2019 · Shift 1 · Kinematics

Speed stays constant, but velocity is a vector, so merely turning changes it. The initial and final velocity vectors have equal length $v$ and the angle between them equals the angle $\theta$ swept at the centre, so $\Delta \vec{v}$ is the base of an isosceles triangle of equal sides $v$, giving $|\Delta \vec{v}|=2v\sin \frac{\theta }{2}$. With $v=10\text{m/s}$ and $\theta =60^{\circ }$, $|\Delta \vec{v}|=2\times 10\times \sin 30^{\circ }=10\text{m/s}$.