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Relatively simple question, I think...
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Now, here is a relatively simple physics question that I can't seem to explain *to myself* satisfactorily. Imagine you board a bus, en route to (fill in the blanks). Halfway through the journey, you feel bored. So you take out a pebble from your pocket (don't ask me how it got there), and you throw it to the other side of the bus. The pebble falls in a arc, obvoiusly. So you pick it up again, but this time, you throw it out the window. Now, the pebble doesn't just fall in an arc. It's falling in an arc, and moving to the left (assuming the bus is travelling to the right). To put it in another way: Assuming the bus is travelling to the right, and I'm standing at one side of the bus, and throwing the pebble to the other side. If the pebble is thrown within the bus, it falls straight ahead. But if it's thrown out of the bus, its falling towards the left of the bus. So why does it fall like that? From what my teacher used to say, the pebble is moving from one system to another, that's why it doesn't fall straight like it is within the bus when it's outside the bus. What's bothering me is: both initial conditions are the same. The pebble is thrown with the same force, from a moving object at the same speed. The only thing different that I can think of the wind resistance (thus the strong winds felt at the window), but it's negligible, considering the mass of the pebble. I thought of relativity as well... In a sense that different observers will perceive the same event differently. The above scenario was for observers in the bus. But what if you were standing outside the bus? If the pebble was thrown within the bus, the observer outside will see the peeble falling down to the right(bus moving right). But when the pebble is thrown out, it travels in a straight arc. So does Einstein's first two postulates for relativity fit as a part of this argument? Or can simple mechanical explanation be sufficient to explain this? Or am I just thinking too much?? Either way, I need answers. Plz help! |
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Re: Relatively simple question, I think...
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by
on 2008-05-25 13:46:08 |
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The observable behavior of the pebble falling inside the bus is due to gravity. When thrown out the window, the observable behavior is overcome by the force of friction due to the high velocity crosswind. In other words: The difference is that no wind blows inside the bus. 
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Re: Relatively simple question, I think...
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| But it seems to me that friction caused by the wind wont affect the object much, especially when the object is quite heavy. Besides, isnt the surface area of the pebble exposed to such winds quite small? So isn't wind resistance a negligable factor? |
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Re: Relatively simple question, I think...
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Im not quite of a help , but I think this is my answer to your problem (hope it's right :P) : The pebble , you and the other people in the bus are actually static . Because the bus moves , anything in the moving bus will move along with it . That's why when the pebble is thrown in the bus , it acts the same way as you throw the pebble under normal condition (like you throw it for fun at a park) . If the pebble is thrown outside of the bus (bringing along the speed of the bus) , and because Earth is technically , next to static (rotates very slow XD) , the pebble will fall the way it happen if you throw in within the bus . Right ? No . Like Gendou said , wind will slow it down drastically . Why ? Because every object dropping from the sky with the help of gravity will increase it's speed 'till the wind resistance = the falling object + gravity . To be exact , all object will have their respective , unasisted top speed and will NEVER exceed it's own top speed (unless if it's been propelled by other force e.g: Bullet fired out of a gun) . Now , apply it to the pebble . The pebble's weight is light . Therefore , it does not have a high top speed (unassisted , only with the help of gravity) . The reason why the pebble reacted the way you mentioned in the bus because there is no wind involved to slow it down , and because there is no wind to slow it down , Inersia takes over and will keep on gaining momentum = to the bus speed . Once outside , the pebble (with it's initial speed of a bus speed) is on it's own (no bus assistance) and will have to go against the wind resistance at the speed of a bus speed (which basically , everyone knows that a pebble speed is far too slow than of a bus speed) . And because it's top speed (freefall speed , only gravity assisting) is extremely low , and it's freefalling in the horizontal direction , like freefalling from the sky , the gravity = wind resistance law involves (I'm not sure the name of that law because I learn my science in non-English . So , translating it is really difficult >.< . Hope Gendou knows it , or I'll have to find the term myself =_=*) . Now , pebble + gravity > wind resistance . To equalize it , wind resistance fights back by drastically reduce the object speed 'till it returns to it's maximun freefall top speed , thus pebble + gravity = wind resistance . And the answer , the pebbles falls behind REAL quick XDD . And the arc will remain an arc (the same way you saw when it is thrown in the bus) with one extra direction , if the bus is going to the right , then it will go left + the arc and vice versa . Why arc remains an arc ? The arc is done by you ^^ in a different direction . Therefore , the wind resistance does next to nothing to the pebble's arc (and it's speed , done by you) , except bending it to the right XD . Hope I explain it right :P . I'm good in science (physics and chemistry ..... not biology XD) but the reason why I don't hang around here because I sucked at explaining XDD . If you want a better explaination , watch Mythbusters XDD . It very much help me in transating most of my non-English science knowledge to English science ^^ and at the same time , learn new things and some humor ^^ . |
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Re: Relatively simple question, I think...
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Put simply: The wind has a much bigger effect than you think it does.
10 more years!
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Re: Relatively simple question, I think...
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by
on 2008-05-26 20:48:45 |
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The faster an object moves (relative to the air), the higher the wind resistance. This is because each molecule in the air will impact at a higher speed on average. If you doubt this, put your hand out the window of a moving car and feel it for yourself.
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