Because of the brake, the car starts to make uniform deceleration linear motion at the speed of 10m / s. if the average speed in the first second is 9m / s, what is the displacement of the car in the first 7S?

Because of the brake, the car starts to make uniform deceleration linear motion at the speed of 10m / s. if the average speed in the first second is 9m / s, what is the displacement of the car in the first 7S?

Let the speed of the car at the end of the first second be v1. From the meaning of the question. V = V0 + V12, the solution is: V1 = 2. V-v0 = 8m / s, so: a = v1-v0t = 8-101m / S2 = - 2m / S2. The time required for the car to reduce its speed to zero after shutting down the engine is: T = △ VA = 0-10-2s = 5S. Because 7S > 5S, the car stops after shutting down the engine for 5S. Therefore, the displacement of the car within 7S after shutting down the engine is: x = v02t = 5 × 5m = 25m. A: within the first 7S of the car The displacement is 25m