There is an observer on the train platform. When the train starts, he just stands at the front end of the first carriage. After the train starts, he accelerates evenly. After 4 seconds, the first carriage passes the observer, and the time for the whole train to pass the observer is 20 seconds. If each carriage is equal in length and the length of carriage connection is not taken into account, we can find: (1) how many carriages are there in this train; (2) the last 9 carriages pass the observer The time experienced by the observer

(1) Let the acceleration of the train be a, then the length of a carriage L = 12at2 = 8A. The displacement of the train x = 12at2 = 12 × a × 400 = 200A in 20s, the number of trains n = XL = 200a8a = 25

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1. If a person throws an object horizontally towards the rear of the train in the train carriage, what can another person standing on the ground platform see the trajectory of the object
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