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2015-09-10T22:41:09+05:30

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U = 60 km/sec
v = 55 km/sec
t = 20 sec
let us assume that a uniform deceleration is resulted by firing the retro engines.
   a = (v - u)/ t = - 5/20 = - 0.25 km/s²
distance travelled in the first 20 sec. = s = u t + 1/2 a t²
   s = 60 * 20 - 1/2 * 0.25 * 20² = 1, 150 km.

After 20 sec. the velocity of the spaceship is constant at 55 km/sec.  Hence, the distance travelled is = 55 * 20 = 1, 100 km

The total distance travelled in 40 sec. = 2, 250 km

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