Question

a train crosses a platform 100 m long in 60 seconds at a speed of 45 km/hr. the time taken by the train to cross an electric pole is: 8 sec, 52 sec, 1 minute, data inadequate

Explanation:

Step1: Convert speed to m/s

Speed \( v = 45\space km/hr \). To convert to \( m/s \), use \( 1\space km = 1000\space m \) and \( 1\space hr = 3600\space s \). So \( v = 45\times\frac{1000}{3600}=\frac{45000}{3600}=12.5\space m/s \).

Step2: Find length of train

When crossing platform, distance covered is \( \text{length of train}(L) + \text{length of platform}(100\space m) \) in time \( t = 60\space s \). Using \( \text{distance} = v\times t \), we have \( L + 100 = 12.5\times60 \). Calculate \( 12.5\times60 = 750 \), so \( L = 750 - 100 = 650\space m \).

Step3: Time to cross electric pole

When crossing electric pole, distance covered is length of train \( L = 650\space m \). Time \( t=\frac{L}{v}=\frac{650}{12.5}=52\space s \).

Answer:

52 sec