CBSE Class 11-science Answered

Make a v-t graph. The area under the v-t graph gives us the distance, which is also equal to average speed *total time = 25 *20 = 500m

 

v-t graph will have three parts, one for the acceleration, second for the constant speed and third for deceleration. Since, the acceleration gives us the slope of v-t graph, so the slope of acceleration curve and deceleration curve is +5 and -5 respectively. Now, if you will assume that the time for which the body remains in acceleration as x sec, then the time taken for deceleration is also x sec. (By symmetry of the curve). Hence, the time for which the body moves with a constant speed is (25-2x) sec. Now, you can calculate area under the graph in terms of x, with the slope known as 5 and base as x, (25-2x) and x respectively. You will get a quadratic equation and on solving it you will get x = 5 sec, which gives the time for which the body moves with constant speed as 15 sec.