A dynamometer D is attached to two bodies of masses M=6 kg and m=4 kg. Force F=20 N and f=10N are applied to the masses as shown. The dynometer reads

Answer

C

Explanation

When F is applied to 'M'

F - kx = Ma kx - f = ma `(F - kx)/(kx - f) = (Ma)/(ma) = (M)/(m)`
Reading of dynamo meter `(kx)_(i) = (mF + Mf)/(M + m)`
after interchanging ther force `(kx)_(f) = (mf + mF)/(M + m)`
`((kx)_(i)))/((kx)_(f)) = (mF + Mf)/(mf + MF) = (2 + 10)/(1 + 20) = (12)/(21) = (4)/(7)`