# How long does it take for a 100 m long train moving at a speed of 36 km / m to completely pass a 200

How long does it take for a 100 m long train moving at a speed of 36 km / m to completely pass a 200 m long tunnel? Can a train be considered here as a material point?

V = 36 km / h = 10 m / s.
L = 100 m.
S = 200 m.
t -?
The time of the complete passage of the tunnel by the train will begin from the moment when the first car begins to enter the tunnel.
The train will completely pass the tunnel when its last point (the last car) leaves the tunnel. At this time, the first car will be at a distance of L = 100 m from the end of the tunnel.
The first car must travel the distance L + S.
The time for the train to complete the tunnel t will be determined by the formula: t = (L + S) / V.
t = (100 m + 200 m) / 10 m / s = 30 s.
In this case, the train cannot be considered a material point. Since its dimensions are commensurate with the length of the tunnel.
A body can be considered a material point when its dimensions are many times less than the distance over which the movement takes place.
Answer: the time of the complete passage of the tunnel by the train is t = 30 s.

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