Originally posted by: exdeath
Originally posted by: smack Down
Originally posted by: exdeath
Originally posted by: smack Down
Originally posted by: ElFenix
Originally posted by: sao123
Originally posted by: ElFenix
the whole thing depends on how much force the treadmill can put through the wheels.
if you've got 10,000 lbs of thrust from the turbines, you're going to need 10,000 pounds of force coming through the wheels to keep the plane from moving.
free spinning wheels will allow on a transfer of force up to a maximum of the friction generated between the wheels, the lubricant, and the center axle. Which in this case is not significant.
which is why this whole problem is stupid. for the plane to take off, the wheels are going to be spinning much faster than the conveyor belt. but the question has assumed that the wheels and conveyor belt are spinning at exactly the same rate. no normal wheel will do that. the only way it'd happen is if there is lots of friction in the wheel. so the problem has already assumed that the necessary friction is there. so the plane won't take off, but only because the problem is stupid and has nothing to do with reality.
The wheel doesn't need friction.
If the wheel was on the treadmill with out the plane it would not spin. Nothing is connected to the axle so not torque is applied.
The wheels on the plane spin so a force must be applied to the torque must be applied to the wheels otherwise they would not spin. Friction itself can not make the wheels spin. The kinetic energy of the spinning wheel increased the kinetic energy of the plane must decrease.
Have you ever pushed a car with the engine off or dead? The wheels rotate even though there is no power applied to the wheels right? The wheels rotate because the rolling friction of the bearings is less than the sliding friction of the rubber and asphalt... the wheel has no choice but to rotate.
Take a heavy box of books placed on thick carpet, and another equally heavy box on top of the first box. Push the top box only. Because the friction between the bottom box and the carpet is greater than the friction between the two boxes, the boxes will slide against each other before they both move together. This is exactly the same transfer of motion that rotates the wheels in your car when you push on the car body in neutral, and is the reason the wheels rotate on a plane when the engines push against the body of the plane. There is no power applied to the wheels. Friction is the only force causing the wheels to rotate.
Are we clear on that part at least?
Which part of a wheel with no axle on a treadmill don't you understand. For reference that is not that same as a car.
I didn't posted it because this whole thread has assumed it but we are talking about the case with infinite static friction so the wheel can not slide.
We are talking about a plane on a treadmill. A wheel simply sitting on a treadmill will sit stationary with the treadmill, yes. You understand that at least. Now attempt to expand your understanding if you are willing.
On a plane however, you have axles suspended on bearings that connect the wheel to the plane.
In order to get torque, a rotational force, from a linear force, that linear force must be directed at some point other than the rotational axis.
In the plane you have two forces on the wheel.
The first force is the force of the engine pushing the axle forward, edge on, though the bearings like this: (axle) <---- force of engines on plane. This force pushes the axle assembly forward without causing torque because it is directed directly into the center of rotation of the wheel. If there were no other forces involved, or if the plane was sitting on a frictionless surface, the wheels would stay still and be carried by the axle and the plane, and there would only be linear movement. The wheels would not spin.
But we have another force, static friction on the bottom of the wheel. As the plane pushes the wheel horizontally through its axis of rotation, the wheel, in contact with some surface on only one side, experiences friction in the opposite direction that the plane is trying to push it. This is on the outside edge of the tire, and this friction force is not directed towards the axis of rotation but is tangent to the perimeter of the tire. Thus a net torque results causing the wheel to rotate, even though there is no power directly applied to the axle.
Don't bother coming back to this thread until you understand the physics behind a simple wheel first.
Facebook
Twitter