Question

An athlete completes one round of a circular track of radius $R$ in $40$ sec. What will be the magnitude of displacement at the end of $2$ min $20$ sec?

• A. Zero
• B. $2R$
• C. $2\pi R$
• D. $7\pi R$

Answer 1

The correct option is B $2R$

Total time of motion is 2 min 20 sec = 140 sec
Time period of circular motion = 40 sec
In 140 sec. athlete will complete$=\frac{2\pi R}{40}\times 140=3.5\left(2\pi R\right)=3.5\text{revolutions}$
So, he will be at diametrically opposite point i.e. Displacement $=2R$.