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# I can't comprehend: A Question About Gravity Laws.

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Hi there. My name is Jordan, I'm seventeen and I'm new to this board. Hopefully this question will only require a simple explanation...we'll see.

Now consider you're in a moving train that's going around 100 mph. With you is a toy helicopter that you can have hover above the ground and move with some sort of remote. Now if you were to have it hover inside the train it would stay in the same place despite the fact that the train is moving at high speeds, correct?

Now, instead of being inside the train, let's say you were outdoors on some sort of train platform that's exposed (think Indiana Jones and the Last Crusade...); then the previous example would dictate that you could indeed hover this toy helicopter while standing on this platform without having it collide with the train...but could you? If you were to hover the helicopter OUTSIDE of the moving outdoor platform going 100 mph wouldn't the train pass it by? Why then would this not happen if you did it inside?

Unless I'm WAY off....

I'd appreciate an answer to this.

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man ... i think we might be using the same brain ... ha ha I was just thinking about that .... still i have not found an answer ... but I have a freind that is a teacher and a chemist ... when he gets home from work i will ask him. i will get back to you soon

~steph~

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Thanks! It confuses me so!

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This is not so much a question of gravity as it is of friction. Gravity will influence the vertical distance the toy maintains from the ground. The friction from the surrounding atmosphere will influence the horizontal motion. Inside the train the flow of air around the toy is (for simplicity) static, in relation to the air flowing around the train car. Gravity is (and always will be) accelerating “down.” So the toy appears, to the observer, to be hanging steady.

In your example of the Indiana Jones mining car ride, the toy would appear to hang steady so long as it remains below the lip of the car, where the air resistance is minimal. If the toy rises above the lip while the car is in motion, the friction from the air it is passing through would “hold” it back while the observer moves forward.

Clear as mud?

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Somewhat Yes. I thought wind resistence would come into play. Although I meant the scene with the Young Indiana Jones, played by River Phoenix, at the beginning of The Last Crusade - not the mining scene in Temple of Doom!

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ok i called him ... he said....wind resistance if the helipcopter is inside the train, the train is already breaking the wind making the air inside of the train stand still but if the helipcopter has to break the wind itself that will slow it down... its inertia

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this hurts my brain, i always hated physics.

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1. Toy in train car (*Assumes a vacuum inside)

Hotizontal Speed of Train = 100 mph

Horizontal Speed of Toy Before Hovering = 100 mph

Horisontal Speed of Toy While Hovering = 100 mph

Becasue it is in a vacuum, there is nothing to slow it down so the toy will stay in place since it is moving at the same horizontal speed as the train. If you take that example farther by changing the speed of the train, eventually the toy will hit either end, how quickly depending on its starting position, change in speed and length of the car.

It should also be noted that these numbers would hold true for a platform in a vacuum.

2. Toy on train car (*Assumes some air resistance)

Hotizontal Speed of Train = 100 mph

Horizontal Speed of Toy Before Hovering = 100 mph

Horizontal Speed of Toy While Hovering = 100 mph - drag

With this example, you would subtract the drag (figured out by calculating windspeed and factoring in anything that reduces or increases airflow over the toy) from the Horizontal Speed of the Toy which will eventually slow it to a stop and then accelerate it to whatever speed the drag is. Oh, that is only for a directly opposite wind, cross winds and tail winds have opposite effects, but those are a little more rare when you are at speeds of 100 mph.

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this is weird, i'm sorry everyone.

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Thanks, Space Moose. That's what I was trying to say.

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