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(i) A man is standing between two vertic...

(i) A man is standing between two vertical walls 680 m apart . He claps his hands and hears two distinct echoes after 0.9 second and 1.1 second respectively. What is the speed of sound in the air?
(ii) A door is pushed , at a point whose distance from the hinges is 90 cm , with a force of 40 N. Calculate the moment of the force about the hiages.

Text Solution

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(i) Velocity = `("Distance travelled by sound")/("Time takes")`
`v=(2d)/(t)`
`d_1=(vt_1)/2 and d_2-(vt_2)/2`
`d=V/2(t_1+t_2)`
`v=(2d)/(t_1+t_2)`
`=(2xx680)/(0.9+1.1)=(2xx680)/2=680"ms"^(-1)`
`v = 680 "ms"^(-1)`
(ii) The moment of a force M = `Fxxd`
`d=90 cm = 0.9m`
Hence, moment of the force `=40xx0.9=36` Nm
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