A physics problem in grade one of senior high school When a train starts to move in a straight line with uniform acceleration from static state, an observer stands at the front end of the first carriage. He measures that it takes 10 seconds for the first carriage to pass through him, and it takes 30 seconds for all trains to pass through him. Assuming that each carriage is equal in length, the total number of trains is

A physics problem in grade one of senior high school When a train starts to move in a straight line with uniform acceleration from static state, an observer stands at the front end of the first carriage. He measures that it takes 10 seconds for the first carriage to pass through him, and it takes 30 seconds for all trains to pass through him. Assuming that each carriage is equal in length, the total number of trains is

The length of each carriage is s, and there are n carriages in total
S=1/2a*10^2
nS=1/2a*30^2
N = 9 can be obtained by dividing the two equations