Something amazing to tell you concerning physics and motion

[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.

[cdcox]

Originally Posted by orange

As I mentioned earlier, perfect rolling requires infinite friction.

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

Actually, perfect rolling generates zero frictional force opposing the thrust of the jets.

[jAZ]

Originally Posted by MediaCenterJunkie

So the group paid $27 ($9 each), and he has $2 in his pocket, equaling $29.

What happened to the other dollar?

The group paid $27 INCLUDING the $2 in his pocket... and got $1 each back.

Nothing happened to the $1 but bad math.

[orange]

Originally Posted by arc

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.

Again, friction forces don't really enter into it. You WANT high friction, but whatever friction you have is going to apply equally to the wheels and the treadmill.

The operative variable is resistance to the treadmill spinning freely.

[jAZ]

Originally Posted by Sully

If he pocketed $2 they paid $28.

No, they paid $27 ($25 + $2 = $27).

He gave each back $1 (x3) = $30 total.

Nothing to see here.

[MagicHef]

Originally Posted by arc

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.

You realize the friction in question is in the wheel bearings, not the tires, right? Besides, those are made of MagicHefium, so no worries there.

[Frosty]

Originally Posted by orange

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.

I'm not sure you are adding up your forces correctly. The force of the jets is moving the plane forward. The wheel is only spinning because of its frictional contact to the pavement. However, that is partly from the downward force of the weight of the jet along with a component of the forward force. However, the whole force isn't transferred to the wheels, so the treadmill can't counteract all of it.

[jAZ]

Originally Posted by Sully

You are overthinking it!!!!!
$30-$2 is NOT $27!!

$30 - $2 (kept) - $3 returned = $25 (meal price).

The original question is flawed (presumably by design).

[orange]

Originally Posted by cdcox

Actually, perfect rolling generates zero frictional force opposing the thrust of the jets.

The friction I am talking about is the friction between the wheels and the ground/treadmill. This is the coefficient of friction.

For dragging you want low. For rolling you want high.

[MagicHef]

I think orange and I have fundamental differences in how we each view this scenario, so keep that in mind as you are discussing this with us.

[jAZ]

Originally Posted by Baby Lee

not 27+2=29 [an irrelevant sum].

This.

[orange]

Originally Posted by arc

I'm not sure you are adding up your forces correctly. The force of the jets is moving the plane forward. The wheel is only spinning because of its frictional contact to the pavement. However, that is partly from the downward force of the weight of the jet along with a component of the forward force. However, the whole force isn't transferred to the wheels, so the treadmill can't counteract all of it.

Yes, it is. The wheels MUST keep up with the rest of the plane or the plane rotates forward and plunges nose first into the ground.

For the wheels to keep up, they MUST roll or slide along the ground (unless you have a hopping plane). Once you generate enough speed to produce lift, you can then, well, "lift" them off the ground.

[cdcox]

Originally Posted by orange

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

Force of jets - rolling resistance = large positive number

(large positive number) / (mass of jet) = acceleration

Lets say after 1 second, the jet is moving at 1 mph.

Jet speed 1 mph and gaining. Wheel speed 1 mph. Treadmill speed = 0.

Now start your treadmill. Run it at 100 mph.

Jet speed 1 mph and gaining. Wheel speed 101 mph. Treadmill speed = 100 mph.

Double the speed of the tread mill, same thing. Meanwhile, the jet just keeps on picking up speed relative to the ground.

The tread mill is just making the jet wheel free spin. The force of the engines cannot practically be overcome by spinning the wheels faster. Just like you can't break the rope by spinning the treadmill faster under the shopping cart. You just can't generate the force.

[orange]

Originally Posted by MagicHef

I think orange and I have fundamental differences in how we each view this scenario, so keep that in mind as you are discussing this with us.

I guess you're still using a motor to spin your treadmill?

[Baby Lee]

Originally Posted by MagicHef

I think orange and I have fundamental differences in how we each view this scenario, so keep that in mind as you are discussing this with us.

I think we figured that out long ago, when cdcox pointed out the ambiguity I applauded.

You view the hypothetical as inviolable, the way you read the wording the airplane CANNOT be allowed to move forward or the conveyor is failing in it's [ambiguously] stated mission. If a plane can't move forward it can't take off. And if it takes wheels with Herculean rolling resistance sufficient to counteract the thrust of 4 jumbo jet engines so be it.

However, hypotheticals generally assume [unless stated otherwise] negligible or no friction [rolling resistance]. And in that case the interactions of the wheels and the conveyor are akin to those of molecules flowing under a flying craft or in the water or ice under skids or pontoons on a thusly equipped craft, and they are irrelevant in the dominant force of forward thrust by the engines.

[orange]

Originally Posted by cdcox

Force of jets - rolling resistance = large positive number

(large positive number) / (mass of jet) = acceleration

Lets say after 1 second, the jet is moving at 1 mph.

Let's not. The jet NEVER MOVES. The treadmill begins spinning at the precise same instant that the wheels start to turn.