- #106
Well, it does depend on the relationship between the ratio of the radii and the angle of the slope. What if the slope increases to 60 degrees?DaveC426913 said:Cool.
No, they will not roll.
The thread, the radii and the two touch-points of the wheels together form a trapezoid.
The first thing that rolling would theoretically result in is the lengthening of the longest side (the line between the two touch-points).
That trapezoid cannot be deformed and still maintain symmetry.
To elaborate, static equilibrium is maintained when the torque due to gravity and the torque due to the tension in the string are in opposite directions. The former is non-zero as long as ##\theta < 90^o##. The latter becomes zero when the radius of the inner wheel is large enough so that the position vector from the contact point to the point of application of the tension is horizontal. This happens when ##r/R=\cos\theta##. For ##r/R>\cos\theta## the two torques are in the same direction and there can be no equilibrium. Note: Torques are calculated using the point of contact of the wheel with the incline as origin.haruspex said:Well, it does depend on the relationship between the ratio of the radii and the angle of the slope. What if the slope increases to 60 degrees?
DaveC426913 said:Cool.
No, they will not roll.
The thread, the radii and the two touch-points of the wheels together form a trapezoid.
The first thing that rolling would theoretically result in is the lengthening of the longest side (the line between the two touch-points).
That trapezoid cannot be deformed and still maintain symmetry.
Not so sure now.
View attachment 223687
DaveC426913 said:Cool.
No. But that is not a genuine solvable problem. It is an advertisement for “advanced physics tutors”. It is designed to disempower and confuse students to the point where they subscribe to a study program. Now being advertised on PF.A.T. said:Can this spool roll down the incline without slipping, as the problem statement suggests?
But, if I pull the string upwards...? Will it roll?A.T. said:Can this spool roll down the incline without slipping, as the problem statement suggests?
View attachment 223703
Found here:
http://www.chegg.com/homework-help/questions-and-answers/spool-move-spool-rests-top-incline-made-two-uniform-disks-radius-r-mass-m-connected-horizo-q24458961
You're right ... In a rigid body, you can't consider 2 simultaneous "centers of rotation" at the same time (so, a point from which all the other body points motion can be described as circles around that "point")... such body would be no rigid.andrewkirk said:I promised some pictures earlier, and I've finally made them. Here are three pictures, showing a wheel rotating around a point of contact with the ground. The first is a perfectly circular wheel, the second is a wheel with the bottom flattened (eg a tyre compressing under the weight of the load) and the third is a polygon.
We see that, if the wheel rotates around the foremost point of contact in the direction of travel (marked) through a non-zero angle, it all works OK for the polygon and the flattened wheel, but not for the perfect circle, which has to go below the floor.
The answer arrived at in the thread above is that there is no rotation through any angle about that point. Rather, the statement that the wheel is rotating about that point is just a description of the instantaneous relative velocities of points on the wheel.
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Define upwards, and what you mean by pull. Consuming string at the fixed anchor point will move the spool up the ramp. Feeding string out from the fixed anchor point will allow the spool to roll. Raising the anchor point to change the angle between the string and the slope will make a big difference at some point.jmolmo said:But, if I pull the string upwards...? Will it roll?
If it's an string, I think it's clear what I mean by pull. If you mean how much pull, then image you do very, very little at first (0.01N) ... and then more and more.Baluncore said:Define upwards, and what you mean by pull. Consuming string at the fixed anchor point will move the spool up the ramp. Feeding string out from the fixed anchor point will allow the spool to roll. Raising the anchor point to change the angle between the string and the slope will make a big difference at some point.