Question

As in the determination of the various unknown forces in the free body diagram involves the use of vectors. So one of the vector law is commutative law and it is valid for the cross product of two vectors. (Commutative law: PxQ = QxP; for two vectors P and Q)

(a) True

(b) False

The question was asked in unit test.

I need to ask this question from Problems Involving Dry Friction topic in division Friction of Engineering Mechanics

Answer 1

The correct answer is (b) False

The explanation is: This statement is wrong. It is not possible unless we apply a negative sign to the RHS of the equation. That is PxQ = -(QxP). It is because, if you curl your wrist from one vector towards another vector, the thumb projected will give the direction of the cross product. Thus if you reverse the direction, the negative sign is necessary.