The car moves forward on the straight road at the speed of 10m / s, Suddenly, I found a bicycle moving in the same direction at a speed of 4m / s in front of me. The car immediately turned off the accelerator and moved in a straight line with a constant deceleration acceleration of 6m / s. The car just couldn't touch the bicycle. How far was the car from the bicycle when the accelerator was turned off?

The car moves forward on the straight road at the speed of 10m / s, Suddenly, I found a bicycle moving in the same direction at a speed of 4m / s in front of me. The car immediately turned off the accelerator and moved in a straight line with a constant deceleration acceleration of 6m / s. The car just couldn't touch the bicycle. How far was the car from the bicycle when the accelerator was turned off?

In the following two solutions, the motion is treated as uniform acceleration in the opposite direction
Solution 1: the critical condition for a car to miss a bicycle is that the speed is equal when the car and bicycle meet
The time needed to reduce the vehicle speed from 10m / s to 4m / S is
T = (10-4) / 6 = 1 second
In one second,
The distance of the bicycle is X1 = 4 * 1 = 4m
The forward distance of the car is x2 = (10 ^ 2-4 ^ 2) / (2 * 6) = 7m
When the throttle is closed, the distance between the car and the bicycle is x = x2-x1 = 7-4 = 3M
Solution 2: take the bicycle as the reference frame, the initial speed of the car is 6 m / s to avoid hitting the bicycle, and the speed of the car needs to be reduced by 6 m / s, which takes 1 second
X = (6 ^ 2) / (2 * 6) = 3M

If a car advances at a speed of 10m / s along a straight road at a constant speed, and the acceleration of-0.2m/s2 is used for emergency braking, the displacement of the car passing through in 1 min after braking is ()
A. 240mB. 250mC. 260mD. 90m

Let V 0 = 10 m / s, a = - 0.2 M / S2. Let v = V 0 + at, t = V − V 0A = 0 − 10 − 0.2 = 50 s, that is, when t = 50 s, the speed of the car just decreases to zero, then the displacement within 60 s after braking is equal to the position within 50 s after braking