## Will the train's cargo slam into the front of the boxcar?

EL PROBLEMA

A train travelling at 45MPH comes to a stop in 31 feet, experiencing a uniform deceleration.
Inside one of its flat-bed boxcar's is a steel block that sits 9 feet from the front wall of its container.

THE QUESTION

If the load indeed strikes the wall of the car (which I've determined it does), please calculate its speed relative to the train as the moment of impact.

RELEVANT VALUES
• Coefficient of static friction = 0.3
• Coefficient of kinetic friction = 0.25
• Initial velocity of the train = 45MPH = 20.1168m/s
• Stopping distance of the train = 31' = 9.4488m
• Block's distance from the forward wall = 9' = 2.7432m
MY WORK SO FAR
I'm half asleep at the moment so I'm not exactly confident about my answers but so far I've determined that if the train had to stop within 31 feet, it's deceleration would be -21.414ms-2. Again, that's probably not even close.

And I'm not really sure where to go from here...
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 Recognitions: Gold Member Homework Help Science Advisor Your acceleration of the train looks good, in a direction opposite it's motion . Now you need to calculate the acceleration of the crate, with respect to the ground, using newton 2 after first identifying the net force acting on it. Noe find the relative acceleration, and the relative speed in the manner you used to find the trains acceleration . You don't have to convert to metric it's just another source for math errors.
 I'm still confused. I guess my problem is finding the relative acceleration. Is it simply equal to the sum of the acceleration of the train and the kinetic friction?

## Will the train's cargo slam into the front of the boxcar?

I also don't understand when I'm supposed to use static friction or kinetic friction in this problem.

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