A and B students practice the 4 × 100 meter relay on the straight track. They have the same maximum speed when running. B starts to run with all his strength from a standstill A. B two students practice 4 times 100 relay on the straight. They have the same maximum speed when running. B runs with all his strength from a standstill. It takes 25 meters to reach the maximum speed, This process can be regarded as uniform variable speed movement. Now a is running to B with the maximum speed. B is waiting for the opportunity to run out in the relay area. If B is required to run at 80% of the maximum speed when taking over the baton, Q: 1: what is the distance S1 that B needs to run in the relay area? 2: what is the distance S2 that B needs to run from a?

A and B students practice the 4 × 100 meter relay on the straight track. They have the same maximum speed when running. B starts to run with all his strength from a standstill A. B two students practice 4 times 100 relay on the straight. They have the same maximum speed when running. B runs with all his strength from a standstill. It takes 25 meters to reach the maximum speed, This process can be regarded as uniform variable speed movement. Now a is running to B with the maximum speed. B is waiting for the opportunity to run out in the relay area. If B is required to run at 80% of the maximum speed when taking over the baton, Q: 1: what is the distance S1 that B needs to run in the relay area? 2: what is the distance S2 that B needs to run from a?

Let B's time from rest to maximum velocity be t, maximum velocity be V, and B's acceleration be a, then 1 / 2 * a * T ^ 2 = 25, v = at, t = V / a 1. When reaching 80% of the maximum velocity, B takes T2 = 80% v / a = 0.8t, and when reaching 80% of the maximum velocity, B moves S1 = 1 / 2 * a * / (0.8t) ^ 2 = 0.64 * (1 / 2 * a *)

In the 100 meter race, a classmate accelerates in the first 30 meters and moves at a constant speed in the last 70 meters. The speed at the end of the race is 7 m / s, and the test result is 15 s. then how many M / S is his speed when he runs to the midpoint, and how many M / S is his average speed in the first 30 meters?

(1) It is known from the title that he moves at a constant speed at the back 70m, and the speed at the end is 7m / s, so the speed at the back 70m is 7m / s, and the speed at the middle point is 7m / S; (2) the speed at the back 70m is v = 7m / s, from v = ST: the time for the back 70m is t = s, the back v = 70m7m / S = 10s, ∵ t total = 15s, the time for the first 30m