Two forces with equal magnitudes $F$ act on a body and the magnitude of the resultant force is $F/3$. The angle between the two forces is
${\cos ^{ - 1}}\left( { - \frac{{17}}{{18}}} \right)$
${\cos ^{ - 1}}\left( { - \frac{1}{3}} \right)$
${\cos ^{ - 1}}\left( {\frac{2}{3}} \right)$
${\cos ^{ - 1}}\left( {\frac{8}{9}} \right)$
The resultant of two vectors $\overrightarrow P $ and $\overrightarrow Q $ is $\overrightarrow R .$ If $Q$ is doubled, the new resultant is perpendicular to $P$. Then $R $ equals
A body is moving under the action of two forces ${\vec F_1} = 2\hat i - 5\hat j\,;\,{\vec F_2} = 3\hat i - 4\hat j$. Its velocity will become uniform under an additional third force ${\vec F_3}$ given by
A truck travelling due north at $20 \,m/s $ turns west and travels at the same speed. The change in its velocity be
If the magnitude of sum of two vectors is equal to the magnitude of difference of the two vectors, the angle between these vectors is ........ $^o$