Question

A train starting from rest travels the first part of its journey with constant acceleration a, second part with
constant velocity v and third part with constant retardation a, being brought to rest. The average speed for

the whole journey is 7v/8
The train travels with constant velocity for ...of the total time-

(A)3/4
(B)7/8
(C)5/6
(D)9/7

8
6​

Answer 1

Explanation: answer is 3/4

Let a be the constant acceleration (same magnitude for acceleration and deceleration phases but opposite signs). Let the total time of travel be T

Expressingt' in terms of T and t.

T=t'+t'+t

t'=(T-t)/2

The equation for train's first part of journey:

x(1)=1/2at'^2

=1/2a(T-t/2)^2.

The equation for train's second part of journey:

x(2)=vt

The equation for train's second part of journey:

x(3)=vt'-1/2at'^2

avg speed =7v/8.= [x(1)+x(2)+x(3)]÷T

(14/8)T-T=t

t=3/4T.

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Answer 2

Answer:

this is the answer for the question