- #1

fireemblem13

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## Homework Statement

A train is decelerating with initial velocity 10 m/s. What should be the acceleration of the train for stopping in 3 seconds at distance of 8m.

## Homework Equations

## The Attempt at a Solution

L=vt + at^2 /2

a=2(L-vt)/t^2

Substituting in, gives a=-4.89

Apparently, this is wrong.

Another problem.

Two friends are traveling by train, and they have nothing to do. They decide

to measure the speed of train. The first one starts counting rails of the railroad. He knows

that the length of one rail is 10 meters, and it takes him 3 minutes to count 156 rails. He

calculated that the speed is equal to 31.2 km/hour. The second one starts counting pillars

along the railroad. He has counted 32 pillars, and this takes him 3 minutes (he knows that

the distance between pillars is 50 meters). He calculates that the speed is 32 km/hour. Who

makes a mistake, and what is the speed of the train?

I said the second person was wrong, since he was counting the distances between pillars, so the first person must be right and the velocity must be 31.2 km/h. Apparently this is wrong too.

Any help appreciated.