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On an open ground, a motorist follows a track that turns to his left by an angle of $60^{\circ}$ after every $500 \mathrm{~m}$. Starting from a given turn, specify the displacement of the motorist at the third, sixth and eighth turn. Compare the magnitude of the displacement with the total path length covered by the motorist in each case.
PhysicsMotion In Two Dimensions
Solution:
1885 Upvotes Verified Answer
In this question, the path is a regular hexagon $A B C D E F$ of side length $500 \mathrm{~m}$. In Fig,


Let the motorist start from $A$.
Third turn
The motorcyclist will take the $3^{\text {rd }}$ turn at $D$. Displacement vector at $D=A D$
Magnitude of this displacement $=500+500$ $=1000 \mathrm{~m}$
Total path length from $A$ to $D=A B+B C+C D$ $=500+500+500=1500 \mathrm{~m}$
Sixth turn
The motorcyclist will take the $6^{\text {th }}$ turn at $A$.
$\therefore \quad$ Displacement vector is null vector.
Magnitude of this displacement $=500+500$ $=1000 \mathrm{~m}$
Total path length $=A B+B C+C D+D E+E F$ $=500+500+500+500+500+500=3000 \mathrm{~m}$
Eighth turn
The motorcyclist takes the $8^{\text {th }}$ turn at $C$.
$\therefore \quad$ Displacement vector $=A C$, which is represented by the diagonal of the parallelogram $A B C G$.
$$
\therefore \sqrt{\left[(500)^2+(500)^2+2 \times(500) \times(500) \cos 60^{\circ}\right]}
$$
$$
=\sqrt{\left[(500)^2+(500)^2+250000\right]}=866.03 \mathrm{~m}
$$
$\tan \beta=500 \sin 60^{\circ} \%\left\{500+500 \cos 60^{\circ}\right]$
$$
\begin{aligned}
&=(500 \sqrt{3} / 2) /\{500(1+1 / 2)\}=1 / \sqrt{3} \\
&=\tan 30^{\circ} \text { or, } \beta=30^{\circ}
\end{aligned}
$$

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