A car start moving along a line first with acceleration a=2 starting f...
Let the total distance travelled by the car be D. The total time of motion is 10s; therefore the average speed is (D/10) m/s. We are given the average speed as 3.2 m/s. So (D/10) = 3.2 or D= 32 m.
Now the car starts from rest, accelerates for some time at 2 m/s^2, then moves at a uniform speed and finally decelerates at 2m/s^2 to come to rest. As the car had started from rest, the time for acceleration and deceleration is the same. Let the time the car accelerates and decelerates be t1 and the time it moves at a uniform speed be t2. We get: 2*t1+ t2 = 10.
Now the distance it travels while accelerating is 2*t1*t1/2, this is the same as the distance it travels while decelerating. The speed it has after t1 s is 2*t1. The distance travelled at the uniform speed is 2*t1*t2.
This gives us 2*t1*t1/ 2 + 2*t1*t2 + 2*t1*t1/ 2 = 32.
Now 2*t1+ t2 = 10
=> t1 = (10 - t2)/ 2
We substitute this in 2*t1*t1/ 2 + 2*t1*t2 + 2*t1*t1/ 2 = 32
=> 2*t1*t1 + 2*t1*t2 = 32
=> 2*[(10 - t2)/ 2] ^2 + 2*[(10 - t2)/ 2]*t2 = 32
=> [(10 - t2)/ 2] ^2 + [(10 - t2)/ 2]*t2 = 16
=> (10 - t2) ^2 + 2*10*t2 - 2*t2*t2 = 64
=> 100 + t2^2 - 2*10*t2 + 2*10*t2 - 2*t2*t2 = 64
=> 100 + t2^2 - 2*t2*t2 = 64
=> t2^2 = 36
=> t2= 6 or -6
As time cannot be negative, so we have t2= 6 s
Therefore the car moves uniformly for 6 s