Science Something amazing to tell you concerning physics and motion - Page 30

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.

**cdcox** · 06-04-2009, 10:55 PM

prhom --

Does the fact that the planes wheels are spinning fast somehow counteract the thrust generated by the jet engine?

That force will cause the plane to accelerate relative to the ground unless there is some counteracting force. Short of the wheel bearing seizing that force does not exist. Unless you want to suspend Newton's 2nd law.

prhom · 06-04-2009, 10:55 PM

Quote:

Originally Posted by cdcox

Now you are depending on the bearing burning up and seizing in order to keep the jet from taking off. I think that violates the spirit of the brain teaser.

No, because as the extra force pushing the plane forward is translated into faster wheel rotation the treadmill matches it.

orange · 06-04-2009, 10:57 PM

Quote:

Originally Posted by prhom

I'm going to bed after this because frankly, I'm exhausted now. Since the treadmill has an unlimited capacity to accelerate it could theoretically continue accelerating until the friction in the wheel bearings was greater than the acceleration force of the plane. Again, in the real world everything would burn up in this ridiculous scenario pretty fast.

The wheels/treadmill will not accelerate indefinitely. In the real world, both have some friction which will produce great drag at higher rotational velocity. The wheels will reach a maximum speed - matched by the treadmill - and the system will run in equilibrium as long as the jet fuel lasts.

**cdcox** · 06-04-2009, 10:57 PM

Quote:

Originally Posted by orange

The rocket cars. Already mentioned.

How is a rocket car different from a match box car with a hand pushing it? Do you have a video of a rocket car spinning on a tread mill?

**cdcox** · 06-04-2009, 11:00 PM

Quote:

Originally Posted by orange

speed of jet wheels (depends on) - speed of treadmill (equals wheel speed) =
speed of treadmill (equals wheel speed) + speed of jet (determined by force balance) - speed of treadmill (equals wheel speed)

~~speed of jet wheels (depends on) - speed of treadmill (equals wheel speed)~~ =
~~speed of treadmill (equals wheel speed)~~ + speed of jet (determined by force balance) - ~~speed of treadmill (equals wheel speed)~~

0 = speed of jet (determined by force balance)

The force balance on my jet makes it go 5 mph at a particular instant. What are you going to set your treadmill at?

Joe Seahawk · 06-04-2009, 11:01 PM

Wheels do not matter when the jet is moved by the thrust of the jets.. the wheels and the treadmill are nearly irrelevant. This seems very simple to me.. Is something wrong with me?

It's almost like saying a jet couldn't take off if it had skis instead of wheels and the substrate was ice.

**cdcox** · 06-04-2009, 11:01 PM

Quote:

Originally Posted by prhom

I'm going to bed after this because frankly, I'm exhausted now. Since the treadmill has an unlimited capacity to accelerate it could theoretically continue accelerating until the friction in the wheel bearings was greater than the acceleration force of the plane. Again, in the real world everything would burn up in this ridiculous scenario pretty fast.

BTW, Oranges treadmill doesn't have a motor. It free wheels.

orange · 06-04-2009, 11:01 PM

Quote:

Originally Posted by cdcox

How is a rocket car different from a match box car with a hand pushing it? Do you have a video of a rocket car spinning on a tread mill?

The hand pushing the car has to move forward. The rocket engine expels mass out the back. It can produce energy to move the car while standing in place.

prhom · 06-04-2009, 11:02 PM

Quote:

Originally Posted by cdcox

prhom --

Does the fact that the planes wheels are spinning fast somehow counteract the thrust generated by the jet engine?

That force will cause the plane to accelerate relative to the ground unless there is some counteracting force. Short of the wheel bearing seizing that force does not exist. Unless you want to suspend Newton's 2nd law.

See #437, because the treadmill can accelerate its not a static problem. It's a dynamics problem now, the extra force gets counteracted by some magical force propelling the treadmill to MATCH the speed of the wheel. It's tricky because the wheel and treadmill are by definition going the same speed as long as they are in contact with one another. So you can substitute matched speed with matched force. As soon as the plane starts to "speed up" relative to the treadmill the treadmill will respond with added speed from some magical engine that provides that forces. It's that external magical force that makes this a stupid question.

orange · 06-04-2009, 11:02 PM

Quote:

Originally Posted by cdcox

BTW, Oranges treadmill doesn't have a motor. It free wheels.

Less parts to burn up. It gets all the energy it needs to perfectly match the wheels - from the wheels (ultimately from the jet engines).

prhom · 06-04-2009, 11:04 PM

Quote:

Originally Posted by Joe Seahawk

Wheels do not matter when the jet is moved by the thrust of the jets.. the wheels and the treadmill are nearly irrelevant. This seems very simple to me.. Is something wrong with me?

Unfortunately, yes, because the treadmill can accelerate. If the treadmill is not powered or runs at a fixed speed then the plane might have a chance depending on how fast the treadmill can go and how much power the plane has.

orange · 06-04-2009, 11:04 PM

Quote:

Originally Posted by Joe Seahawk

Wheels do not matter when the jet is moved by the thrust of the jets.. the wheels and the treadmill are nearly irrelevant. This seems very simple to me.. Is something wrong with me?

It's almost like saying a jet couldn't take off if it had skis instead of wheels and the substrate was ice.

If you think it's a very simple question - yes there's something wrong with you. Try googling it. Baby Lee mentioned one site that has 300 PAGES of comments.

And there are many more sites.

And the ice in your example is incorrect. It's not moving relative to the rest of the world. The ground under the plane in the problem is. Mud is a better example.

prhom · 06-04-2009, 11:06 PM

Quote:

Originally Posted by orange

Less parts to burn up. It gets all the energy it needs to perfectly match the wheels - from the wheels (ultimately from the jet engines).

That's where you are incorrect orange. The treadmill must have an external force propelling the treads to counteract the plane engine thrust.

**cdcox** · 06-04-2009, 11:06 PM

Quote:

Originally Posted by Joe Seahawk

Wheels do not matter when the jet is moved by the thrust of the jets.. the wheels and the treadmill are nearly irrelevant. This seems very simple to me.. Is something wrong with me?

No. There is something wrong with me for keep trying to come up with different explanations in an attempt to teach.

orange · 06-04-2009, 11:09 PM

Quote:

Originally Posted by prhom

That's where you are incorrect orange. The treadmill must have an external force propelling the treads to counteract the plane engine thrust.

The plane engine thrust affects the wheels and the treadmill equally. It doesn't have to be counteracted.

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.
Posts: 15,986

06-04-2009, 10:55 PM	#436
cdcox www.nfl-forecast.com Join Date: Sep 2000 Casino cash: $-488231	prhom -- Does the fact that the planes wheels are spinning fast somehow counteract the thrust generated by the jet engine? That force will cause the plane to accelerate relative to the ground unless there is some counteracting force. Short of the wheel bearing seizing that force does not exist. Unless you want to suspend Newton's 2nd law.
Posts: 46,032

06-04-2009, 11:01 PM	#441
Joe Seahawk This place entertains.. Join Date: Aug 2000 Location: Seattle Casino cash: $5494435	Wheels do not matter when the jet is moved by the thrust of the jets.. the wheels and the treadmill are nearly irrelevant. This seems very simple to me.. Is something wrong with me? It's almost like saying a jet couldn't take off if it had skis instead of wheels and the substrate was ice.
Posts: 5,514