Science Something amazing to tell you concerning physics and motion - Page 18

Jenson71 · 06-04-2009, 10:15 AM

This blows my mind.

Okay, say you have two ball bearings. You hold one in your hand and drop it. The other you put in a gun, point it straight forward, as level as the one in your hand, and pull the trigger, sending it hundreds of yards.

Which one lands first?

The one dropped in your hand right? RIGHT!?!?

No, actually they both hit the ground at the same time.

This is because the vertical component of motion is the same for both objects. They both fall vertically the same distance at about 10 meters per second squared (gravity, actually more like 9.8).

But Jenson, Jenson! What about the horizontal component of the gun-fired ball bearing? Actually, when air resistance is small enough to ignore as it is in this case (being that the bearings are exactly alike), the horizontal and vertical components of a projectile's velocity are completely independent of one another.

And, in this experiment, the gun-fired bearing is constantly falling from the moment it is fired. Yes, it looks like it is holding up in a straight line, but, it's actually falling and it's just hard to see with our eyes. And again, it is falling at exactly the same speed as the ball bearing you released in your hand -- the speed of gravity.

Post more cool science things in this thread.

I think I will later post Aristotle vs. Galileo and gravity, for a little history spin on science.

**Kyle DeLexus** · 06-04-2009, 03:48 PM

Quote:

Originally Posted by Jenson71

This blows my mind.

Okay, say you have two ball bearings. You hold one in your hand and drop it. The other you put in a gun, point it straight forward, as level as the one in your hand, and pull the trigger, sending it hundreds of yards.

Which one lands first?

The one dropped in your hand right? RIGHT!?!?

No, actually they both hit the ground at the same time.

This is because the vertical component of motion is the same for both objects. They both fall vertically the same distance at about 10 meters per second squared (gravity, actually more like 9.8).

But Jenson, Jenson! What about the horizontal component of the gun-fired ball bearing? Actually, when air resistance is small enough to ignore as it is in this case (being that the bearings are exactly alike), the horizontal and vertical components of a projectile's velocity are completely independent of one another.

And, in this experiment, the gun-fired bearing is constantly falling from the moment it is fired. Yes, it looks like it is holding up in a straight line, but, it's actually falling and it's just hard to see with our eyes. And again, it is falling at exactly the same speed as the ball bearing you released in your hand -- the speed of gravity.

Post more cool science things in this thread.

I think I will later post Aristotle vs. Galileo and gravity, for a little history spin on science.

Congrats on your middle school graduation! Be sure to get a Transam and have The Stroke playing when you pull into the parking lot on your first day of high school. It would also help your popularity if you wear a denim jacket and REO Speedwagon shirt to match.

**cdcox** · 06-04-2009, 03:48 PM

Quote:

Originally Posted by orange

Why? What relevance has that got?

The problem states that the speed of the treadmill (x) should be set to the speed of the wheels (y).

So that gives the equation x = y.

The speed of the wheels is

y = x + z

where z is the speed of the plane. The speed of the plane is determined by the result of its time history of acceleration. Acceleration is determined by a force balance on the plane. The forces are the thrust of the jets, air resistance and bearing resistance. The force of the thrust is far greater than the other two, so the plane will move forward which makes z positive, let's say 5.

substituting

x = x + 5.

You simply cannot make the treadmill speed and wheel speed the same, since speeding up the treadmill will make the wheels go faster requiring more speed up and so on and so on.

MagicHef · 06-04-2009, 03:49 PM

Quote:

Originally Posted by MediaCenterJunkie

I also think that momentum and inertia may have something to do with it. The original question didn't ask if the plane is moving when the treadmill started up, or if they started at the same exact time and kept the same exact speed throughout.

The question is certainly flawed, even when you don't consider that any of this would never be possible. Momentum and Inertia have everything to do with anything, but I don't believe that you need to consider them to evaluate this specific question.

orange · 06-04-2009, 03:49 PM

Quote:

Originally Posted by bdeg

that is not possible.

Sorry but all physics questions assume ideal cases, then add outside factors. In this case, there are two outside factors - the coefficient of friction between the ground and the wheels; and the other resistances to the plane and treadmill moving.

The coefficient of friction is largely irrelevant. We've already assumed it's high enough to require the wheels to roll. And it will effect the wheels and the treadmill equally.

All we need to keep the plane motionless is that the resistance to the treadmill rolling be equal or lower than the other resistance (air resistance, basically) to the plane moving forward. If that resistance to the treadmill is low enough, any slipping due to imperfect friction will cause the treadmill to rotate FASTER than the wheels and the plane will back up.

MagicHef · 06-04-2009, 03:50 PM

Quote:

Originally Posted by arc

Then where does the force of the engines go? If they are thrusting at so many pounds against the air, there has to be an equal force against the plane to keep it from not moving. That means you are saying that the treadmill is exerting the same force against the plane as the engines, but since the wheels are spinning, how can that be? They would have to have tremendous friction to match the engines' force.

Yep.

Frosty · 06-04-2009, 03:52 PM

Quote:

Originally Posted by MagicHef

Yep.

So what kind of magic material do you have that would create that much friction against standard rubber tires?

MagicHef · 06-04-2009, 03:52 PM

Quote:

Originally Posted by cdcox

You simply cannot make the treadmill speed and wheel speed the same, since speeding up the treadmill will make the wheels go faster requiring more speed up and so on and so on.

Essentially, the wheels and treadmill would have to go from a velocity of zero to infinite speed instantaneously and simultaneously.

**Buck** · 06-04-2009, 03:52 PM

Quote:

Originally Posted by MagicHef

The question is certainly flawed, even when you don't consider that any of this would never be possible. Momentum and Inertia have everything to do with anything, but I don't believe that you need to consider them to evaluate this specific question.

Well I'm glad i wasted half of my day in this thread

orange · 06-04-2009, 03:53 PM

Quote:

Originally Posted by arc

Then where does the force of the engines go? If they are thrusting at so many pounds against the air, there has to be an equal force against the plane to keep it from not moving. That means you are saying that the treadmill is exerting the same force against the plane as the engines, but since the wheels are spinning, how can that be? They would have to have tremendous friction to match the engines' force.

As I mentioned earlier, perfect rolling requires infinite friction.

We don't need to go that far, though. See my last post.

**cdcox** · 06-04-2009, 03:54 PM

Quote:

Originally Posted by orange

All we need to keep the plane motionless is that the resistance to the treadmill rolling be equal or lower than the other resistance (air resistance, basically) to the plane moving forward.

No, the bearing friction also has to overcome the thrust of the jets.

MagicHef · 06-04-2009, 03:55 PM

Quote:

Originally Posted by arc

So what kind of magic material do you have that would create that much friction against standard rubber tires?

This is not a question of material properties, which is certainly getting into details that I don't think are necessary for an inherently impossible situation. Assume that there is a material "MagicHefium" that makes the amount of friction necessary possible.

orange · 06-04-2009, 03:55 PM

Quote:

Originally Posted by cdcox

You simply cannot make the treadmill speed and wheel speed the same, since speeding up the treadmill will make the wheels go faster requiring more speed up and so on and so on.

Yes you can. Equal and opposite reaction. The same force that causes the wheels to spin causes the treadmill to spin at the exact same time.

MagicHef · 06-04-2009, 03:56 PM

Quote:

Originally Posted by MediaCenterJunkie

Well I'm glad i wasted half of my day in this thread

You're welcome.

Frosty · 06-04-2009, 03:58 PM

Quote:

Originally Posted by MagicHef

This is not a question of material properties, which is certainly getting into details that I don't think are necessary for an inherently impossible situation. Assume that there is a material "MagicHefium" that makes the amount of friction necessary possible.

It most certainly makes a difference. For friction forces to match the thrust of the jet engines, the tires would last a millisecond before melting down, at which time the jet would begin moving down the runway, scrapping along on its struts.

orange · 06-04-2009, 03:59 PM

Quote:

Originally Posted by cdcox

No, the bearing friction also has to overcome the thrust of the jets.

It must or the wheels will rip off. If the wheels don't roll or slide, they have to do something.

06-04-2009, 10:15 AM
Jenson71 fides quaerens intellectum Join Date: Oct 2003 Location: United States Casino cash: $8330900	Something amazing to tell you concerning physics and motion This blows my mind. Okay, say you have two ball bearings. You hold one in your hand and drop it. The other you put in a gun, point it straight forward, as level as the one in your hand, and pull the trigger, sending it hundreds of yards. Which one lands first? The one dropped in your hand right? RIGHT!?!? No, actually they both hit the ground at the same time. This is because the vertical component of motion is the same for both objects. They both fall vertically the same distance at about 10 meters per second squared (gravity, actually more like 9.8). But Jenson, Jenson! What about the horizontal component of the gun-fired ball bearing? Actually, when air resistance is small enough to ignore as it is in this case (being that the bearings are exactly alike), the horizontal and vertical components of a projectile's velocity are completely independent of one another. And, in this experiment, the gun-fired bearing is constantly falling from the moment it is fired. Yes, it looks like it is holding up in a straight line, but, it's actually falling and it's just hard to see with our eyes. And again, it is falling at exactly the same speed as the ball bearing you released in your hand -- the speed of gravity. Post more cool science things in this thread. I think I will later post Aristotle vs. Galileo and gravity, for a little history spin on science.
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