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Old Jan 25th 2009, 05:20 PM   #1
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Kinematics question

Hey I was just wondering if someone could help me with the working on this...

A student, standing on the platform at Cambridge railway station, notices that the first two carriages of an arriving train pass her in 2.0s, and the next two in 2.4s. The train is decelerating uniformly. Each carriage is 20m long. When the train stops, the student is opposite the last carriage. How many carriages are there in the train?

The answer is 8 carriages, I'm just stuck with the working

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Old Jan 25th 2009, 08:21 PM   #2
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Do you know how to write the formula for the distance that an object has traveled at time t when it is moved by a constant acceleration. The general formula has three constants in it: The constant acceleration (unknown in this problem), the objects velocity at time t = 0 (unknown in this problem ) and its distance at time = 0. You can write this equation for an object which is the point at the end of the first two cars. You can write it for an object that is the point at the end of the first 4 cars. You have enough information to to put numbers in each equation at the a given time. That gives two simultaneous equations to solve for the two unknowns, acceleration and initial velocity. Then you have enough information to do the rest of the problem.
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