
#1
Sep1207, 05:21 PM

P: 275

1. The problem statement, all variables and given/known data
005 (part 1 of 1) 10 points A 23car train standing on the siding is started in motion by the train's engine. There are 4 cm of slack between cars. The cars are 9 m long. The engine is tightly connected to the first car and moves at a constant speed of 22 cm/s. How much time is required for the pulse to travel the length of the train? Answer in units of s. 2. Relevant equations i used the t = d/v 3. The attempt at a solution i calculated the distance of the train to be 9m * 23 cars + 4cm slack* 22 cars slack is between = Distance and then converted to cm and used the Velocity as 22 cm/s and solved for T is this the correct approach 



#2
Sep1207, 05:40 PM

P: 275

are my calculations for the distance of the train correct?
and is it correct to use the formula t=d/v to solve for the time the pulse travels 



#3
Sep1207, 05:43 PM

P: 275

i get my calculations for the length of the train to be 20,788 cm
so to solve for the time i took 20,788 cm / 22cm/s is this correct? 



#4
Sep1207, 05:54 PM

Mentor
P: 39,720

Time for pulse to travel a length
I don't think the length of the train cars matters. I believe only the slack matters. What answer do you get if you make that assumption?




#5
Sep1207, 05:55 PM

P: 275

i get 4 seconds.... but why would the lenght of the train cars not matter? seems strange to me




#6
Sep1207, 05:57 PM

P: 275

4 seconds was the correct answer ... i jsut am confused y the lenght of the cars doesnt matter




#7
Sep1207, 05:57 PM

Mentor
P: 39,720





#8
Sep1507, 12:57 AM

HW Helper
P: 1,664

Realistically, "rigid" objects don't move all in one piece. The sudden application of force to one end of the object is communicated by a "shock" travelling at the speed of sound in the material through the body of the object (the molecular bonds are disturbed at one end of the object first, which then disturbs the bonds of molecules further along the object, and so on...). This speed would be somewhere upward of 1000 m/sec, so it takes several milliseconds for the trailing end of a railcar to respond to the fact that the leading edge was yanked forward. In the real world, the length of the cars *would* matter, but the added total delay to the travel time of the pulse would be proportionally small. I also agree with your result of 4 seconds (I was amused that they chose the values so that the 22's would cancel). The total delay from the "shock" traveling through the bodies of the railcars adds perhaps about another 0.1 second. 


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