Airplane on a treadmill

[Scn64]

If you hold a hot wheels car on a moving treadmill, will the treadmill force your hand and the hot wheels car back, or will your hand keep the car in one place? You'll probably agree that you'd be able to keep the car in one place. Well, in that example, the treadmill would be moving faster than the car, right? So why can't it force the car back? It's because your hand, which isn't affected by the treadmill, is holding the car in place. Now if you pushed the car forward it would move forward. The wheels would be spinning at the speed of the treadmill + the speed you're moving the car. The jets on a plane just act as the "hand" on the plane.

 

[BeardofPants]

The question doesn't pose any external force. Plane go one way. Threadmill go other. Both same. V - V = 0.

<edit> the "hand" can do all it wants - the threadmill will match the velocity as stated by the question.

 

[Scn64]

The question doesn't pose any external force. Plane go one way. Threadmill go other. Both same. V - V = 0.

<edit> the "hand" can do all it wants - the threadmill will match the velocity as stated by the question.

The jets are the external force in that they are not affected by the ground. If you had a giant fan in front of the plane, that could work against the jets. Moving ground, however, is not going to affect them.

When I originally read this question on another board I thought the same thing as you. After someone explained it to me and I had a while to think about it, I realized I was wrong.

 

[BeardofPants]

But you are then ignoring what the question is posing. If the question stated that the wheel speed velocity was exactly matched by the treadmill velocity, then it becomes a question of wheel velocity/friction/blah blah. However, the question refers to the velocity measured by the total movement of the plane - engines and all - then the velocity would be zero.

 

[K62]

Now if you pushed the car forward it would move forward

If you push the car forward, and it moves forward then the car is moving faster than the treadmill.

The treadmill, which turns in the opposite direction that the plane is facing, is capable of precisely matching the speed of the airplane.

The treadmill will match the speed of the car exactly, so it CAN NOT MOVE FORWARD. lol

 

[Scn64]

But you are then ignoring what the question is posing. If the question stated that the wheel speed velocity was exactly matched by the treadmill velocity, then it becomes a question of wheel velocity/friction/blah blah. However, the question refers to the velocity measured by the total movement of the plane - engines and all - then the velocity would be zero.

The only reason I mention wheels is because when people read this question they often times think of a car that relies on its wheels to move it forward. This, of course, is not the case with a plane. If the plane is moving forward at 50 mph, the treadmill will be moving backward at 50mph. This wont cause the plane to stop because the jets are pushing against the air.

I can think of one more example. You are on a moving treadmill wearing rollerblades. There is a horizontal rope next to the treadmill that is fastened to two walls in front and back of you. Even when the treadmill is moving, you can grab onto this rope and pull yourself forward.

The jets can act on the air and move the plane forward. It doesn't matter how fast the treadmill is moving because it can't apply force against the force of the jets. The jets push against air and the treadmill is ground. I don't expect to convince you right away. All that I ask is you think more about what I'm trying to say.

 

[Scn64]

If you push the car forward, and it moves forward then the car is moving faster than the treadmill.

It doesn't have to be. When you're holding the car in one place the car most definitely is not moving faster than the treadmill. Yet still, the treadmill isn't able to force it back.

 

[K62]

The only reason I mention wheels is because when people read this question they often times think of a car that relies on its wheels to move it forward. This, of course, is not the case with a plane. If the plane is moving forward at 50 mph, the treadmill will be moving backward at 50mph. This wont cause the plane to stop because the jets are pushing against the air.

I can think of one more example. You are on a moving treadmill wearing rollerblades. There is a horizontal rope next to the treadmill that is fastened to two walls in front and back of you. Even when the treadmill is moving, you can grab onto this rope and pull yourself forward.

The jets can act on the air and move the plane forward. It doesn't matter how fast the treadmill is moving because it can't apply force against the force of the jets. The jets push against air and the treadmill is ground. I don't expect to convince you right away. All that I ask is you think more about what I'm trying to say.

lol, I give up.

If you pull your rope and move forward, you will be moving faster forward than the treadmill is moving backwards. SAMMMMEE SPEEEED

 

[K62]

It doesn't have to be. When you're holding the car in one place the car most definitely is not moving faster than the treadmill. Yet still, the treadmill isn't able to force it back.

Right! And the car, relative to the ground/air isn't moving either!

 

[BeardofPants]

*sigh* That's not what the question is asking. It doesn't matter about that stuff. The question says that the velocity of the treadmill will match the velocity of the (whole) plane. No matter what. Point period polkadot. Wheels/engines don't matter. There's no total velocity. The plane is stationary because that is what the question states. The threadmill will match the plane. Exactly.

 

[BeardofPants]

lol, I give up.

If you pull your rope and move forward, you will be moving faster forward than the treadmill is moving backwards. SAMMMMEE SPEEEED

Right. There's no velocity being generated (inertia). It doesn't matter that the wheel speed will be different from the velocity generated by the engines. The forward motion will be matched by the backward motion. At the same speed.

 

[Scn64]

*sigh* That's not what the question is asking. It doesn't matter about that stuff. The question says that the velocity of the treadmill will match the velocity of the (whole) plane. No matter what. Point period polkadot. Wheels/engines don't matter. There's no total velocity. The plane is stationary because that is what the question states. The threadmill will match the plane. Exactly.

I've seen this same question discussed on many boards and it always works the same way. At first most people give the same arguments you're giving. Gradually, they start to realize what's really happening in the situation posed by the question. I don't have any more arguments for you, I would just be repeating myself.

 

[A.B.Normal]

*sigh* That's not what the question is asking. It doesn't matter about that stuff. The question says that the velocity of the treadmill will match the velocity of the (whole) plane. No matter what. Point period polkadot. Wheels/engines don't matter. There's no total velocity. The plane is stationary because that is what the question states. The threadmill will match the plane. Exactly.

Where does it say the plane is stationary?

If you were to interpret "speed" as the forward movement of the plane .Then an increase in "speed" would mean the plane is moving forward and thus will gain lift.
#

Physics. The rate or a measure of the rate of motion, especially:

1. Distance traveled divided by the time of travel.
2. The limit of this quotient as the time of travel becomes vanishingly small; the first derivative of distance with respect to time.
3. The magnitude of a velocity.

By those defintions the plane "is" moving forward and thus will be able to take off.

 

[A.B.Normal]

Right. There's no velocity being generated (inertia). It doesn't matter that the wheel speed will be different from the velocity generated by the engines. The forward motion will be matched by the backward motion. At the same speed.

Regardless if the plane is moving forward at 5mph and the treadmill is moving backward at 5mph,the freakin plane is still MOVING FORWARD j/k.

 

[BeardofPants]

Wrong. Newton's first law of motion states thusly: "Newton's first law of motion predicts the behavior of objects for which all existing forces are balanced. The first law - sometimes referred to as the "law of inertia" - states that if the forces acting upon an object are balanced, then the acceleration of that object will be 0 m/s/s."

http://www.glenbrook.k12.il.us/GBSSCI/PHYS/CLASS/newtlaws/u2l3a.html

That being the case, no velocity = no wind. No wind = no lift. No lift = no take off. Simple.

Scnwotever - you're still misreading the question. The wheels on the plane can be travelling at 4x the speed of the plane for all that it matters - the treadmill will magically adjust to the total velocity of the plane.

 

[BeardofPants]

inertia |i?n?r sh ?| noun 1 a tendency to do nothing or to remain unchanged : the bureaucratic inertia of government. 2 Physics a property of matter by which it continues in its existing state of rest or uniform motion in a straight line, unless that state is changed by an external force. See also moment of inertia . • [with adj. ] resistance to change in some other physical property : the thermal inertia of the oceans will delay the full rise in temperature for a few decades.

 

[A.B.Normal]

But the question says the speeds are the same ,all that means is they are moving away from a central point at equal speed(mph/Kph etc) .If as you say the plane isn't moving then the belt isn't moving either because the question states "Matching speed".In your theory the plane stops moving ,but the belt continues,which doesn't hold true to the question.,

 

[A.B.Normal]

inertia |i?n?r sh ?| noun 1 a tendency to do nothing or to remain unchanged : the bureaucratic inertia of government. 2 Physics a property of matter by which it continues in its existing state of rest or uniform motion in a straight line, unless that state is changed by an external force. See also moment of inertia . • [with adj. ] resistance to change in some other physical property : the thermal inertia of the oceans will delay the full rise in temperature for a few decades.

Where does the ??? say anything about inertia/lift/bananas in pajamas ,it say matching speeds.

 

[BeardofPants]

I'm not saying the plane stops moving - I'm saying it reaches a stationary state cos of newton's first law - ie velocity = zero.

 

[BeardofPants]

Okay now... huh?

Plane ---> 5km .... 5km <--- Treadmill

Zero velocity.