Kriemhild Posted May 30, 2011 Share Posted May 30, 2011 Okay, there's this question that's been causing quite a lot of problems. Basically, you have a block (modeled as a particle) made of material A, that is resting on a disc made of material B, 200 mm away from the center. The disc B is then mechanically rotated at a constant angular acceleration of 0.4 rad s-1. After 9.903s, the block slips. Find the coefficient of static friction between A and B. A pretty lame MS Paint sketch, Now, I could do most of the work here by myself, calculating the acceleration along the normal and tangent axes and whatnot, but there is one thing that's really bugging me. The force X that is acting towards the normal (i.e. towards the center of the disc). Is it part of friction or the reaction from the surface? There's no contact between the materials in that direction, so I don't think it's the reaction, but I don't see how it could be friction either. What also confuses me somewhat is the force F. From what I can deduce, the only forces acting on the block are caused by gravity and from contact with the surface B. Since F is on the tangential axis, my only conclusion was that it was friction, which doesn't seem to make sense to me. Isn't the whole point of friction to oppose the motion? In this case, it seems like friction is the only force CAUSING the motion... Help would be greatly appreciated. Especially within 22 hours, because I have a midterm to worry about that includes this chapter. Quote Link to comment Share on other sites More sharing options...
Balcerzak Posted May 30, 2011 Share Posted May 30, 2011 Okay, there's this question that's been causing quite a lot of problems. Basically, you have a block (modeled as a particle) made of material A, that is resting on a disc made of material B, 200 mm away from the center. The disc B is then mechanically rotated at a constant angular acceleration of 0.4 rad s-1. After 9.903s, the block slips. Find the coefficient of static friction between A and B. Small point, but I assume your mean 0.4 rad s-2, if you're actually dealing with an acceleration problem here. The force X that is acting towards the normal (i.e. towards the center of the disc). Is it part of friction or the reaction from the surface? There's no contact between the materials in that direction, so I don't think it's the reaction, but I don't see how it could be friction either. It's friction. Consider the case of uniform circular motion. Friction would be providing the centripetal force supplying the acceleration needed to constantly change the direction of the velocity, even though its magnitude would be remaining the same. In this slightly more complicated example, you have friction supplying both the direction changing, and the magnitude changing elements, represented by X and F respectively, unless I miss my mark here. What also confuses me somewhat is the force F. From what I can deduce, the only forces acting on the block are caused by gravity and from contact with the surface B. Since F is on the tangential axis, my only conclusion was that it was friction, which doesn't seem to make sense to me. Isn't the whole point of friction to oppose the motion? In this case, it seems like friction is the only force CAUSING the motion... Friction is the source motion... with respect to the outside reference frame. Friction is what keeps the block moving on top of the disc. If the disc were frictionless, the block would stay perfectly still and the disc would pass effortly beneath it. In the rotating frame of the disc however, friction is most certainly opposing the block's motion (with respect to the disc). Quote Link to comment Share on other sites More sharing options...
Blitz Posted May 30, 2011 Share Posted May 30, 2011 Shit Proto having trouble with mechanics,never thought this day of black sun would appear (because I normally would ask YOU if I had problems with Mechanics).Dont worry I couldnt solve it and its a good thing there was a scientist here to help you. Quote Link to comment Share on other sites More sharing options...
Fenrir Posted May 30, 2011 Share Posted May 30, 2011 that drawing kind of resembles a dick if you look at it right. Quote Link to comment Share on other sites More sharing options...
Aleph Posted May 30, 2011 Share Posted May 30, 2011 All smashed in like it went and stepped on a rake, maybe. Quote Link to comment Share on other sites More sharing options...
Kriemhild Posted May 31, 2011 Author Share Posted May 31, 2011 Small point, but I assume your mean 0.4 rad s-2, if you're actually dealing with an acceleration problem here. Yeah, I did, sorry about that. Didn't bother too much with the numbers since I wasn't concerned about them It's friction. Consider the case of uniform circular motion. Friction would be providing the centripetal force supplying the acceleration needed to constantly change the direction of the velocity, even though its magnitude would be remaining the same. Yeah, that's what I initially thought, thanks for confirming it! In this slightly more complicated example, you have friction supplying both the direction changing, and the magnitude changing elements, represented by X and F respectively, unless I miss my mark here. And with X being dependent on the speed, the box would move with respect to the disc when it gets too high. Which is when you calculate the coefficient of static friction by finding the ratio of the magnitude of both components of friction to the constant normal reaction... which is exactly what I did, woo hoo! Friction is the source motion... with respect to the outside reference frame. Friction is what keeps the block moving on top of the disc. If the disc were frictionless, the block would stay perfectly still and the disc would pass effortly beneath it. Now that you gave that example, it makes a whole lot more sense to me. Thanks, Zak! In the rotating frame of the disc however, friction is most certainly opposing the block's motion (with respect to the disc). Yeah, I was thinking more about absolute motion when I saw that but I guess friction doesn't really care about that. Shit Proto having trouble with mechanics,never thought this day of black sun would appear (because I normally would ask YOU if I had problems with Mechanics).Dont worry I couldnt solve it and its a good thing there was a scientist here to help you. Hey, it's not Chemistry, remember? Also, this is the first time I was desperate enough to actually ask over here, because I have a midterm tonight which includes these concepts. Normally, I'd ask YOU first, but that's not really an option anymore unless you actually decide to come online before I sleep. Quote Link to comment Share on other sites More sharing options...
Fenrir Posted May 31, 2011 Share Posted May 31, 2011 All smashed in like it went and stepped on a rake, maybe. or possibly its like melted butter and can't fill a rubber? Quote Link to comment Share on other sites More sharing options...
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