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Q.Define vector product. Explain the properties of a vector product with two examples.

Telangana TsbieTelangana Board of Intermediate Education (Intermediate 1st Year) 2025Subjective· 4mImportance★★★★★
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The vector product of two vectors gives a new vector perpendicular to both, with magnitude |a||b|sinθ; it is anti-commutative and its magnitude equals the parallelogram area formed by the two vectors.

Definition: The vector product (or cross product) of two vectors a⃗\vec{a} and b⃗\vec{b}, inclined at angle θ\theta to each other, is defined as:

a⃗×b⃗=∣a⃗∣∣b⃗∣sin⁡θ n^\vec{a} \times \vec{b} = |\vec{a}||\vec{b}|\sin\theta \, \hat{n}

where n^\hat{n} is a unit vector perpendicular to the plane containing a⃗\vec{a} and b⃗\vec{b}, with its direction given by the right-hand rule: curl the fingers of the right hand from a⃗\vec{a} towards b⃗\vec{b} through the smaller angle; the thumb then points along n^\hat{n}.

Properties:

  1. Not commutative (anti-commutative): a⃗×b⃗=−(b⃗×a⃗)\vec{a}\times\vec{b} = -(\vec{b}\times\vec{a}) — reversing the order reverses the direction of the resultant vector.
  2. Distributive over addition: a⃗×(b⃗+c⃗)=a⃗×b⃗+a⃗×c⃗\vec{a}\times(\vec{b}+\vec{c}) = \vec{a}\times\vec{b} + \vec{a}\times\vec{c}.
  3. Magnitude = area of parallelogram: ∣a⃗×b⃗∣|\vec{a}\times\vec{b}| equals the area of the parallelogram formed with a⃗\vec{a} and b⃗\vec{b} as adjacent sides.
  4. Cross product of a vector with itself is zero: a⃗×a⃗=0\vec{a}\times\vec{a} = 0, since θ=0⇒sin⁡θ=0\theta = 0 \Rightarrow \sin\theta = 0. More generally, the cross product of two parallel (or antiparallel) vectors is zero. …

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