The car moves in a straight line at a speed of 20 m / s and decelerates in a straight line at an acceleration of 5 m / S2 after braking. Then the displacement within 2 s after braking is 0______ ．

The car moves in a straight line at a speed of 20 m / s and decelerates in a straight line at an acceleration of 5 m / S2 after braking. Then the displacement within 2 s after braking is 0______ ．

&If the time from braking to stopping is t = 0 − v0a = − 20 − 5S = 4S ＞ 2S, then the vehicle has not stopped within 2S after braking. From x = v0t + 12at2, the solution is x = 20 × 2 − 12 × 5 × 4m = 30m;

The car moves in a straight line at a speed of 20 m / s, and the acceleration after braking is 5 m / S2. Then the ratio of the displacement of the car passing through 2 seconds after starting braking and 6 seconds after starting braking is 0

The braking time is: T = 205s = 4S, it can be seen that the car stops in 4S, so the displacement after braking for 2S is: X1 = v0t + 12at2 = 20 × 2 − 12 × 5 × 22 = 30m, it has stopped after braking for 6S, so the displacement is: x2 = 202 × 4 = 40m, so the ratio of the displacement after braking for 2s to that after braking for 6S is 30:40 = 3:4 A: the ratio of the displacement after braking for 2s to that after braking for 6S is 3:4

When the acceleration a = 5m / S2 when braking, how long will the vehicle speed be zero ()
A. 15sB. 5sC. 2sD. 1s

According to v = V0 + at, t = V − v0a = − 10 − 5S = 2S. So C is correct and a, B and D are wrong. So C is selected