Senior high school physics topic: Senior high school physics topic: A speedometer sends ultrasonic waves to a car which is 345m away from it, while the car brakes suddenly, and the sound wave speed.

the round-trip time of sound wave is equal, which is half of the braking time of the car when the sound wave meets the car, and the movement of the car is uniformly accelerated with zero initial speed. The ratio of equal time displacement is 1: 3, and it can be obtained that the displacement x1 = (345-325) * 3/4 = 15m before the sound wave meets the car and x2 = 345-325)/4 = before the collision.