A block of √3 kg is attached to a string whose other end is attached to the wall. An unknown force F is applied so that the string makes an angle of 30° with the wall. The tension T is (Given g=10 m s⁻²)

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(4) 20 N. Idea: the wall is vertical, so an angle of 30° 'with the wall' is 30° from the vertical. The string's vertical component is what holds the block up. The block hangs from the knot, so the downward pull at the knot is the full weight: mg=√3×10=10√3 N. Vertical balance at the knot: T cos 30°=10√3 T·(√3)/2=10√3 T=20 N. The horizontal balance would give the unknown force, F=T sin 30°=10 N — not asked for, but a useful check that the numbers are consistent. Reading the 30° from the horizontal instead would give T=(10√3)/(sin 30°)=20√3, which is not among the options.

A block of √3 kg is attached to a string whose other end is attached to the wall. An unknown force F is applied so that the string makes an angle of 30° with the wall. The tension T is (Given g=10 m s⁻²)

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Why the other options are wrong