• Question: Okay so my question is; when travelling in a moving train/car/etc, when we drop something in front of us, why doesnt it fly back when we drop it? eg. a train is travelling north at 100kmph, i drop an empty bottle, it lands right in front of me. Why does it land infront of me instead of flying back south and then hit the floor?

    Asked by enter-scientific-name-here to Aislinn, Padraic, Sinead on 12 Feb 2015.
    • Photo: Padraic Morrissey

      Padraic Morrissey answered on 12 Feb 2015:

      Great question! I’ll answer it better now than I could have in the live chat!

      The answer to your question revolves around what’s known as “relative motion”. When the train is moving, everything inside the train is moving at 100kmph, this includes the bottle and you. When you drop the bottle, it is still moving at the 100kmph because it cant suddenly lose that speed. Because you are travelling at the same speed as the bottle it looks like it falls straight down!

      Another way to look at this would be to think about what happens if you throw a ball out of a car window when it is moving. Does the ball just stop when it hits the ground or does it keep moving for a while before it stops?!

      I hope that helps answer your question! Let me know if you want anything else explain. And don’t forget to vote! 🙂

    • Photo: Sinead Quirke

      Sinead Quirke answered on 12 Feb 2015:


      Padraic has done a great job of answering your question.

    • Photo: Aislinn Coghlan

      Aislinn Coghlan answered on 12 Feb 2015:


      Padraics has done a great job explaining relative motion.

      We you drop the bottle, your hand is actually moving at the same speed as the train. For example, when you see someone on a train or in a car, they are moving along inside it. That’s why if you are inside the train, dropping a bottle would have the same affect as if you were to drop it when you were standing out on a piece of grass.

      In contrast, if you were to drop it out the windows, it would fly backwards because it would be moving slower than the train.