Why is it 90 degrees when it comes to precession? Especially, irrespective of the speed of rotation, it is always 90 degrees, but why? i don't get it.
oh.. uhm... Ok, real life example. The rotor disk of a helicopter, looking from above. In order to make the disk tilt to the left, we need to apply an upward force at the tail most portion of the disk, so that it takes 90 degrees before the force manifests itself and actually has an effect on the rightmost side of the disk, which lifts that part up and tilts it to the left. This is a fact. What I don't understand is why it is 90 degrees.
The torque caused by applying a force is given by: T = r × F where the cross indicates a vector product, and r is the vector pointing from the rotational axis to the point where the force F is applied. The torque T is always at right angles to both r and F, and since torque is the rate of change of angular momentum, the direction of the change in angular momentum is always at 90 degrees to the applied force. Hope this helps.
Ahh.. excellent James R, I think I understand it now. I now get how the physics work out.. still wondering the why question tho Please Register or Log in to view the hidden image!
>> Why is it 90 degrees when it comes to precession? >> angular momentum is always at 90 degrees to the applied force. Why was the question JR !!! The reason why it is at 90 degrees is because precession is a process whereby one spinning axis is trying to spin around (align to ) the same axis of a spin with greater angular momentum. Axises (?sp) are effectively at 90 degrees. It is really simple, the same process is present in magnetic/electric interactions.
Yes, it can, Pete. You can look at individual parts of a rotating object and look at the directions of the velocities of just those parts, and the forces needed to be applied to change those velocities. Right now, I'd prefer not to go into it, since it would require more writing than I have time for at this moment.
Yes, all tops motions and gyroscopes motions are rigid bodies motion. The basic of rigid body motion is given by Newton's laws of motion, where you have an object that is composed of a collection of particles. The distance betwen the different parts of these particles is constrained to be constant (this is what makes the body a rigid body). The solution of Newton's laws of motion give the precessions.
Maybe this diagram will help. Consider the balls as individual particles of the spinning disk. The arrows show the direction of motion of each particle. Note that the particles at the top and bottom are moving left and right, and the particles on the left and right are moving up and down. Now rotate the disk 90 degrees. the top and bottom particles inertia are still carrying them left and right and this pushes on the top and bottom of the disk. this results in the disk wanting to tilt. This tilt is at 90 degrees to the rotatation we applied. (note that the original left and right particles are still moving up and down, and this puts no tilting torque on the disk.
why not tilt 85 degrees? or 110? why 90? I'm pretty sure of the diagram you describe, and i see no reason for it to be 90 degrees (excpet of course for T = r x F, and the fact that it happens in real life)
I think you could argue for 90° from symmetry. If it were not 90°, the gyroscope's path would be unsymmetrical. I think I can also see why it is 90° more directly, but I'm still trying to articulate it. This probably means that I don't quite understand it yet!
I don't know why. I attached it, and when I go to "edit Post" it lists the attached file, but it doesn't show up in the post.
Until, if ever, ny attachment shows up, try imagining this: You have a spinning wheel and each particle has a direction of movement. You are looking at the wheel from along the axis. Each particles movement can be broken up into two components, an up or down component and a left or right component and each component has it own inertia associated with it. . Now let's rotate the wheel along its up and down axis. All the up or down inertias still act through the wheel in such a way as to continue the rotation of the wheel (they add no torque towards tilting the wheel) All you are left with are the right or left inertias, which all work at 90° to the up and down axis and work towards tilting the wheel. It doesn't matter how fast the wheel is spinning or how much we try to rotate it you are always left with these inertias.
