A) 70 \[km{{h}^{-1}}\]
B) 70.7\[km{{h}^{-1}}\]
C) 65 \[km{{h}^{-1}}\]
D) 75\[km{{h}^{-1}}\]
E) None of these
Correct Answer: B
Solution :
From, \[{{v}^{2}}-{{u}^{2}}=2as\] \[\frac{{{80}^{2}}-{{60}^{2}}}{2a}=s\] \[s=\frac{6400-3600}{2a}\] \[=\frac{2800}{2a}\] \[=\frac{1400}{a}\] The middle point of the train is to cover a distance \[\frac{s}{2}=\frac{700}{a}\] From, \[{{v}^{2}}-{{u}^{2}}=2as\] \[{{v}^{2}}-{{60}^{2}}=2a\times \frac{700}{a}\] \[=1400\] \[{{v}^{2}}=1400+3600\] \[v=\sqrt{5000}\] \[=70.7\,km{{h}^{-1}}\]
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