If you were to jump from a moving train at 20 km/h, would it be better to run in the direction of movement before jumping to maintain inertia or in the opposite direction of the train ?
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3$\begingroup$ That depends on what your goal is. If you want to jump farther (and more dangerously), run with the train :) $\endgroup$– tmwilson26Dec 17, 2015 at 13:59
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1$\begingroup$ The goal is not to get hurt $\endgroup$– Al.KaDec 17, 2015 at 14:03
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1$\begingroup$ @tmwilson26 or if your goal is to land on a train next track over, then you want to move such that your sum velocity matches the target train :-) $\endgroup$– Carl WitthoftDec 17, 2015 at 14:41
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1$\begingroup$ Jumping from a moving train is everyday-life? $\endgroup$– Qmechanic ♦Dec 17, 2015 at 18:52
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1 Answer
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Opposite. IN order no not get hurt, you want to be stationary with respect to the ground.
Pro-tip: Don't jump of moving trains! ;)
How badly you hurt yourself will depend on the speed and direction with which you hit the ground. As you jump of the train your veloicty with respect to the ground will be $\vec v_{\mathrm{relative-ground}}=\vec v_{\mathrm{relative-train}}-\vec v_{\mathrm{train}}$. By matching you speed with the train (but in the reverse direction) you will just fall straight down when you exit the train.
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$\begingroup$ What happens if I keep running in the direction of movement of the train even after i have jumped off? $\endgroup$– Al.KaDec 17, 2015 at 14:04
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$\begingroup$ @Al.Ka You vould get badly hurt I would presume, since your speed with respect to the ground would be much higher. $\endgroup$ Dec 17, 2015 at 14:20
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1$\begingroup$ It's good if you are fast enough to cancel the speed of the train. Otherwise, I heard that for practical purposes it is best to face the direction of motion, because it's easier to hold your balance and not get hurt upon landing if you face forwards in the direction of motion relative to the ground. $\endgroup$– LLlAMnYPDec 17, 2015 at 14:23
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$\begingroup$ Reminds me of the MythBusters episode in which they showed that you can, in fact, drive a car at highway speeds right up a ramp onto a truck also moving at highway speeds. The only risk is of damaging the transmission as the wheels come to a screeching halt. $\endgroup$ Dec 17, 2015 at 14:38
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$\begingroup$ As is so often the case, Mythbusters has addressed the question. In this case youtube.com/watch?v=BLuI118nhzc they shot a soccer ball backwards off a moving pickup truck. They were going to have Tony Velachi tossed off by a treadmill, but the insurance folks nixed it. $\endgroup$ Dec 17, 2015 at 16:05