Q#26 (Past Exam Paper – November 2010 Paper 11 Q9 & November 2016 Paper 12 Q10
)
In order that a train can stop safely, it will always pass a signal showing a yellow light before it reaches a signal showing a red light. Drivers apply the brake at the yellow light and this results in a uniform deceleration to stop exactly at the red light.
The distance between the red and yellow lights is x.
If the speed of the train is increased by 20%, without changing the deceleration of the train, what must be the minimum distance between the lights?
A 1.20 x
B 1.25 x
C 1.44 x
D 1.56 x
B 1.25 x
C 1.44 x
D 1.56 x
Solution:
Answer: C.
Since there is a uniform deceleration, we can use the equations of uniformly accelerated motion.
When the initial speed of train is u,
Distance between red and yellow lights = x
Final speed of train = v = 0 [the train stops]
v2 = u2 +2as
0 = u2 + 2ax
u2 ∝ x
[The distance is proportional to the square of the initial speed.]
Now, the initial speed of the train is increase by 20%.
New speed unew = 100% + 20% = 120%
New speed unew = 1.20 u
Since the deceleration is kept constant, u2 ∝ x
New distance xnew ∝ (unew)2 = (1.20 u)2 = 1.44 u2 = 1.44 x
[since u2 ∝ x]
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