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Physics/The path of free-falling motion



If I drop a ball in a moving train, even though the ball is not moving at the speed of train when it is in air, it still behaves normally i.e., it is not drifted back though the train is moving at much faster rate in the forward direction. Can you please explain the reason?

ANSWER: Much faster than what?  The ball?  If the ball is aboard the train, then it's moving at the speed of the train.  Can you explain what would suddenly make it stop moving horizontally at that same speed?  Nothing will if it's still on the train, no wind or anything.  So the horizontal motion is unaffected by dropping it, it just keeps moving with the train.

---------- FOLLOW-UP ----------


Thanks for your reply.

Much faster than what?  The ball?  

I agree that the ball while it was in hand was moving at the speed of train. But when it is released, there is no force acting on the ball to move it forward but the train is moving with a forward velocity. So one would expect that the ball will fall behind the expected point on the floor but it doesn't happen. I could not understand the reason.

Wait, so you have a stationary ball that's being held by someone not on the train who drops it onto the train, or it's held by someone who's on the moving train when they drop it?  In the first situation, it absolutely will end up hitting the train to the rear of the train (not relative the the ground) because the train is moving under it.  If you're referring to a person on the train who drops the ball then the ball is already moving at the speed of the train, not slower.  It only picks up a vertical component of speed as it falls as well.  The ball does not suddenly decelerate to a stop relative to the ground when dropped.  It keeps moving at the speed of the train and hits the train.  What other explanation do you need?  Try it in a car or on a bicycle, dropping a ball (or anything) out the window or off a bike or while running will not cause it to instantly stop relative to the ground.


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Dr. Stephen O. Nelson


I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.


I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.

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