A `100 V` voltmeter having an internal resistance of `20 k Omega` is connected in series with a large resistance `R` across a `110 V` line. What is the magnitude of resistance `R` if the voltmeter reads `5 V`?

Answer

(B)

Explanation

Let the speeds of balls of mass m and `2m` after collision be `v_(1)` and `v_(2)` as shown in figure. Applying conservation of momentum `mv_(1) + 2mv_(2) = m u` & `-v_(1) + v_(2) = (u)/(2)`. Solving we get `v_(1) = 0` and `v_(2) = (u)/(2)`
Hence the ball of mass m comes to rest and ball of mass `2m` moves with speed `(u)/(2). t = (2pir)/(u//2) = (4pir)/(u)`