A car moving at `160 km//h` when passes the mark-A, driver applies brake and reduces its speed uniformly to `40 km//h` at mark-C. The mark are spaced at equal distances along the road as shown below.
At which part of the track the car has instantaneous speed of `100 km//h` ? Neglect the size of the car.

Options

A. A mark-B
B. Between mark-A and mark-B
C. between mark-B and mark-C
D. insufficient information to decide.

Answer

C

Explanation

From `A` to `C` : `(40)^2 - (160)^2 = 2a (AC)`
From `A` to `D` : `(100)^2 - (160)^2 = 2a (AD)`
Here `D` is the point where speed is `100 km//h`.
From above `AD = (13)/(20) (AC) and AB = (AC)/(2)`
We see that AD gt AB`.