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Motion In A Plane

Question
CBSEENPH11018063

When the vector sum of the two forces is perpendicular to their vector difference?

Solution
The two diagonals of a parallelogram formed by two vectors represent the vector sum and vector difference of two vectors. 
When the parallelogram is a square, the diagonals are perpendicular. Then, the two force vectors are represented by the adjacent sides of the square. Hence, the two force vectors which are equal in magnitude and perpendicular to each other have their vector sum perpendicular to their vector difference.