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Find the torque about the origin when a force of 3̂j N acts on a particle whose position vector is 2̂k m.

Asked in NEET 2020 · Torque and moment of a force

• (1) 6̂i N m
• (2) 6̂j N m
• (3) -6̂i N m
• (4) 6̂k N m

Step-by-step solution

Why the other options are wrong

• (1) comes from taking ̂k×̂j=+̂i; the correct cyclic order gives ̂k×̂j=-̂i
• (2) comes from reversing the order to ⃗F×⃗r and then mislabelling the axis; either way the torque cannot lie along ̂j, since it is perpendicular to ⃗F=3̂j
• (4) comes from treating the product as ̂k×̂k; the torque is perpendicular to ⃗r=2̂k, so it cannot lie along ̂k either

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