Another physics oddity

Steve@Advance · Nov 24, 2014

This has been around forever, so if you've heard it, it's still something to think about...

A passenger on a moving train drops an object out the window. What path does it take?

Does it drop straight down, then take a "J" pattern, turning toward the rear of the train as it looses momentum?

Or does it move horizontally, then take a downward turn as it looses momentum?

rustyfarmall · Nov 24, 2014

How fast is the train moving, and how heavy is the object?

Brian Dan · Nov 24, 2014

The object would keep moving forward while dropping down to the ground. The object would hit the ground at the same time as if the train was stopped or standing still. Gravity still takes place as the same rate.
Brian

Dick2 · Nov 24, 2014

That's littering!

But don't worry about it; the guy that bales the hay off the right-of-way will bale it up and some animal will eat it - and the vet bill to extract the item from the animal will be a few thousand dollars.

That's the world that we live in.

JMS/.MN · Nov 24, 2014

All I remember is that when you parachute out of a C130 at about 160 mph, you are facing rear but carried forward about 100 yards, and drop about 100 yards, in the 4 seconds it takes for the chute to open. And if it doesn"t open, you litter the landscape. The rigger says, bring it back, we"ll give you another.

JMOR · Nov 24, 2014

Is the train in a vacuum? :twisted:

NCWayne · Nov 24, 2014

Now sure it's exactly physics, but how about this one. You can use any two size circles you want, but here lets use 24" and 48" in circumference.

Now, mark a point on the rim of the circle that's 48" in circumference. With that mark as a starting point, roll the circle one complete revolution. What you'll find is the circle will move forward 48" in one revolution.

Now do the one that's 24" in circumference the same way. Again, you'll get a forward movement of 24", the same as the circles circumference, for one complete revolution.

Now, attach the two circles together with the marks on the rims lined up together. Now, using the 48" circle as the carrier, roll the assembly one complete revolution and look at the results. What you'll find is that the 24" circle also went one complete revolution, but it actually traveled 48". Conversely, if you use the 24" circle as the carrier, the 48" circle will only travel 24" -vs- 48".

In both cases, the circles, by all rights, shouldn't be able to travel more than the length of their circumference in one complete revolution. What's strange is that you don't change the circumference of either circle, you don't change what 360 degrees, or one complete revolution is, as both circles go around only once in any of the scenarios mentioned. However, when nothing else is done to either circle but attach them together (change their physical location), they basically double, or half, the distance they can travel, and still only complete one revolution.

I read an article about this years ago. Simply saying they are attached together means absolutely nothing in the scientific/mathematic realm as the facts are absolutely nothing changes with the dimensions of either circle to cause the change in forward motion relevant to a revolution as science/math dictates that it should. To, the phenomenon occurs regardless of the measurements used. Based on what the article said, this is one problem that, so far, has absolutely no mathematical solution as to how/why it happens beyond the fact that it does...

MarkB_MI · Nov 24, 2014

The object initially has zero vertical velocity, and its horizontal velocity is the same as the train. So when it is released, it it traveling horizontally, and begins to accelerate vertically at 32 ft/sec<sup>2</sup>. Unless the train is moving slowly or is on a trestle, the dropped object will not have significant vertical velocity when it hits the ground. Its trajectory will follow a downward arc and it will strike the earth in a mostly horizontal direction.

"loses momentum" is a red herring. It doesn't "lose momentum". It is accelerating.

Owen Aaland · Nov 24, 2014

(quoted from post at 12:32:19 11/24/14) Now sure it's exactly physics, but how about this one. You can use any two size circles you want, but here lets use 24" and 48" in circumference.

Now, mark a point on the rim of the circle that's 48" in circumference. With that mark as a starting point, roll the circle one complete revolution. What you'll find is the circle will move forward 48" in one revolution.

Now do the one that's 24" in circumference the same way. Again, you'll get a forward movement of 24", the same as the circles circumference, for one complete revolution.

Now, attach the two circles together with the marks on the rims lined up together. Now, using the 48" circle as the carrier, roll the assembly one complete revolution and look at the results. What you'll find is that the 24" circle also went one complete revolution, but it actually traveled 48". Conversely, if you use the 24" circle as the carrier, the 48" circle will only travel 24" -vs- 48".

In both cases, the circles, by all rights, shouldn't be able to travel more than the length of their circumference in one complete revolution. What's strange is that you don't change the circumference of either circle, you don't change what 360 degrees, or one complete revolution is, as both circles go around only once in any of the scenarios mentioned. However, when nothing else is done to either circle but attach them together (change their physical location), they basically double, or half, the distance they can travel, and still only complete one revolution.

I read an article about this years ago. Simply saying they are attached together means absolutely nothing in the scientific/mathematic realm as the facts are absolutely nothing changes with the dimensions of either circle to cause the change in forward motion relevant to a revolution as science/math dictates that it should. To, the phenomenon occurs regardless of the measurements used. Based on what the article said, this is one problem that, so far, has absolutely no mathematical solution as to how/why it happens beyond the fact that it does...

Works as long as you don't take into account that one or the other also has to slide instead of only rolling.

paul · Nov 24, 2014

Which perspective; as observed from the ground, or as observed from the train?

Paul

Steve@Advance · Nov 24, 2014

And the winner is! Paul!
Both answers can be correct depending on the perspective of the observer.

MarkB_MI · Nov 24, 2014

For the record, Paul did not give an answer. "Depends" is a valid answer to questions about underwear choices among YT'ers, but not to a physics question.

souNdguy · Nov 24, 2014

if i had to guess, i'd say the poster was meaning the item was loosing horizontal velocity, though gaining vertical velocity.

Geo-TH,In · Nov 24, 2014

I don't like any of your answers. Neglecting wind resistance. The path it will take is determined by the vector sum of it's vertical velocity, determined by time and gravity and the horizontal velocity is constant = trains velocity. The shape would be the same as if you were on a cliff and threw a ball horizontally, curve shape.

paul · Nov 24, 2014

Sure, sure, steal my glory, what little I get!

Paul

Anonymous-0 · Nov 24, 2014

Might cause sumone to take a physic.

flying H · Nov 24, 2014

I dont think you have the facts correct. your saying if i have a gear with 24 teath and a gear with 48 teath they both make one revolution at the same time, thats impossible!! While you are rolling your 48 inch ball one rev, the 24 inch ball makes 2 revs. only thing unusual in the 24 inch ball is revolving backwards of the direction of its travel.

wolfman · Nov 24, 2014

Concave down parabolic path, the width(not the correct mathematical term) determined by the speed of the train.

NCWayne · Nov 24, 2014

Look at it like this. Measure the circumference of the tire on your tractor. Now measure the circumference of the rim.

Using that thought as a reference, and my measurements for simplicity, it goes like this. If you rotate the tire alone one revolution it's going to go 48 inches. If you rotate the rim one complete revolution it's going to go 24 inches. If you push the tractor forward on the tire it will go 48 inches, on the rims it will go 24 inches, each distance in just one revolution.

Now, with nothing about the situation changed in regard to the diameter of the tire or rim, physically put the two together. Now, the tractor will move 48 inches forward for one complete revolution of both the rim, and the tire.

That said, the example you use is why a planetary gearbox works. Basically you can attach a 48 tooth gear to a 24 tooth gear. With the two gears together you then input power to the 48 tooth gear, and let the 24 tooth gear be the output, driving the device. The resulting output is 1/2 the input speed, with twice the torque.

The only way this could be possible is for the two gears to be attached together and to make one revolution TOGETHER while the actual circumference on one moves at twice/half the speed of the other one.

I really don't know how else to explain it, but really think about the two examples and you'll understand what I am saying.

GeneMO · Nov 24, 2014

You would also need to take "drag" into account. The friction of the air will affect the arc. Temperature will also have an effect on the density of the air.

Gene

Dr. Walt · Nov 26, 2014

JMOR:

Yeh, a GIANT Electrolux ! LOL

:>)

Another physics oddity

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