Equilibrium and Torque and what to do?

[sv3]

So things were smooth sailing as I started Physics studying and than BAM! - I hit torque and equilibrium. I can do the easy problems that directly ask for torque but I'm having some similar issues:

1) Having a hard time recognizing when I need to consider torque. Can anyone shed some light here on what to look for or what "tells" may be present? (An example problem I didn't consider torque on had a rod sticking out from a wall and there was a wire that attached the end of this rod to the wall (all together made a triangle shape). There was also a mass on the end of the rod. This system was in static equilibrium. The problem asked for Force of tension in the wire and like a dumb, I didn't even consider torque and went about balancing mg against Ft in the y direction. How could I have known I should look at torque?

2) And those questions about kids on a see saw - I actually answered them using center of mass equations and it seems to work out fine. Again, it didn't hit me that I should consider torque. So obviously I'm now pretty worried I'm missing a huge concept and hopefully someone can lead me in the right direction - i do completely understand the equations and manipulating them, etc etc. I think its the concepts I'm missing completely

Thanks in advance,
Steve Torquehater

---

Members don't see this ad.

 

[EkramVahsedi86]

http://www.youtube.com/watch?v=QhuJn8YBtmg&feature=PlayList&p=AD5B880806EBE0A4&index=47

This guy is very philanthropic!

 

[sv3]

cause i need help big time on recognizing when I need to figure out torque and when not to. Talk about not being able to get to the start line!

 

[EkramVahsedi86]

Ok, so you want to know a few trigger words maybe?

Ok, if something is leaning, hanging, swinging, rotating, round, circular, spinning, affixed, etc...

Translational motion is pretty easy. Torque can be difficult. First of all, any time a force is applied to a system/particle/object/projectile etc... it can either change the translational motion or the rotational motion.

I reread your 2nd point and you might already know all you need for the MCAT. The mcat only tests you on torque under the guise of an equilibrium question. If you can do the equilibrium questions just fine, as you said, then you probably already have an intuitive understanding of torque sufficient to ace physics-equilibrium questions. A qualitative understanding is all that is likely to be tested anyways. Torque is much more important in engineering than in medicine and the mcat will only test a very basic 'n easy grasp of torque.

 

[EkramVahsedi86]

More on your first point. Make sure you can "follow" a series of forces. Imagine a billiards ball hitting a pool table. First of all, you have your arm twisting against your shoulder to enable your body parts to move relative to one another. Your arm applies force to the billiards stick. You strike the ball clumsily and offcenter. The right half of the ball gets all the applied force. This will make the ball spin. You have transferred your force into the ball's kinetic rotational energy. Imagine if the spinning ball strikes another ball in such a way that the ball stops its spinning and stop its motion completely! The kinetic energy is transferred from that ball to the new one in the form of a force. Now if that ball does something, then the force is transferred. The system will never stop bouncing around unless the kinetic energy is absorbed by the pool table in the form of friction, which releases heat. Say that the heat warms the air, increasing the kinetic energy of the gas molecules. You have to ingrain in your mind that physics extends throughout everything.

http://www.youtube.com/watch?v=sLZV0Y-VtGw

Here's another video to see the transfer of forces. Any time a force is applied to ANYTHING--it has to pass on to something else, lest it would accelerate. Anything in static equilibrium means that the force has been "passed on" down a long enough string of intermediates so that nothing moves in the system, relative to the static reference frame.

 

[sv3]

those videos were good illustrations. I understand what they were about and I think I just have to look over more problems to understand when to consider torque. I get that a force applied at the center of mass can result in translational but not rotational motion. But, if you apply a force somewhere that is not the center of mass, can you translational along with the rotational that will occur? If you can't, and its jut rotational, then perhaps this will help me recognize when/how to account for translational vs rotational motion.

the kids on a see saw examples are easy but i worry about the type of example i mentioned in my first quote. I get the feeling this stuff can be easy if you "See it the right way" and since math i always find easy, i want to be able to get some "easy" (I use that term loosely) points on the real deal.

thanks very much for your help,

Steve

 

[EkramVahsedi86]

those videos were good illustrations. I understand what they were about and I think I just have to look over more problems to understand when to consider torque. I get that a force applied at the center of mass can result in translational but not rotational motion. But, if you apply a force somewhere that is not the center of mass, can you translational along with the rotational that will occur? If you can't, and its jut rotational, then perhaps this will help me recognize when/how to account for translational vs rotational motion.

the kids on a see saw examples are easy but i worry about the type of example i mentioned in my first quote. I get the feeling this stuff can be easy if you "See it the right way" and since math i always find easy, i want to be able to get some "easy" (I use that term loosely) points on the real deal.

thanks very much for your help,

Steve

Any time I can help, pm me. My MCAT journey as a business major with zero understanding of any science began with physics. I overkilled it and took like 10 pages of notes per chapter for the first 8 chapters, and did EVERY practice problem and redid the ones I got wrong and could probably teach physics 1. If I can share my knowledge in any way with you, then it helps me not feel like I wasted so much time disproportionately studying for something which might be 8 free questions on the mcat PS section of 52 questions! Also, I have watched all of that Khan Academy's physics lectures and all the Walter Lewin 8.01 on youtube. I have enjoyed answering your questions--I regret less my million hours on Isaac Newton's pet sciences.

 

[sv3]

something in common. I have an undergrad in business and a healthcare focused MBA. I also did a med sci undergrad, but i finished it in 2002, meaning my first year sciences were in 1999 (yes im an oldie - 28). So it feels like i am starting from scratch. If me asking questions makes you feel better, I am certain I will help you feel like a billion bucks by the time I'm done with physics!

Actually, I think it started to come together for me yesterday. It's not crystal yet, but if I'm seeing an equilibrium questions and can't solve the Forces part of the equation due to too many variables, I then clue in to use torque to help me (who'd have thunk that.....torque being a friend) b/c if its equilibrium, torque has to be 0 right? Now I just want to be able to clue in quicker. At least that's what the practice problems seem to do. Now I gotto think about when not to worry about torque - perhaps a tricky question on the MCAT will lead you to beleive you need to look at it but don't.

Also, think I answered a question I asked you in my last post. If a wheel is rolling, than that would be translational and rotational at once...and a major headache!

thanks again for being so helpful - I'm sure we'll "chat" soon.

steve