EK torque strategy... is there a better way?

[funklab]

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So Exam Krackers seems to abstractly choose a point of rotation that is not really the point of rotation at all in order to "simplify" the problem. I really have trouble wrapping my mind around their method.

It just seems incredibly counter-intuitive to choose a point of rotation that isn't one.

Is there an easier way, or is the EK method really the best/fastest way to solve an MCAT torque problem?

 

[Kittee]

I rephrased this multiple times to make it more coherent. Here's my best version:

The core point is, it's relative. Just like how two objects in motion can be relative to one (-2 m/s) or the other (+2 m/s) or to a stationary point/observer/object (say, +10 m/s and +8 m/s).

That's why you can pick any point, because when you calculate, you are making calculations relative to this point. Specifically, the lever arm is what changes if you change your reference.

Do one system and keep changing your reference. Notice how the lever arm values adjust to make the final answer the same.

 

[plsfoldthx]

Um... there's always a singular point of rotation. Either they give you the center, the fulcrum, or whatever. Or they will give you the force acting on each one and say they are in equilibrium, in which case you can figure out the center. I dont know what "strategy" you're referring to. torque = force x length from center. that's all.

 

[Kittee]

Um... there's always a singular point of rotation. Either they give you the center, the fulcrum, or whatever. Or they will give you the force acting on each one and say they are in equilibrium, in which case you can figure out the center. I dont know what "strategy" you're referring to. torque = force x length from center. that's all.

I think there's an example in EK Physics where there's a massless board of length held up by a string at L/2. On both ends of the string are two objects hanging.

So, instead of using L/2 as your reference point for the lever arm, you use one edge of the the board, such that when you calculate torque at that point, the value for one of the objects hanging is zero, and you only need to consider Tension at L/2 and the other hanging object at L.

 

[funklab]

I think there's an example in EK Physics where there's a massless board of length held up by a string at L/2. On both ends of the string are two objects hanging.

So, instead of using L/2 as your reference point for the lever arm, you use one edge of the the board, such that when you calculate torque at that point, the value for one of the objects hanging is zero, and you only need to consider Tension at L/2 and the other hanging object at L.

Yup, thats the one. I guess I just gotta practice it a few times...

I hate it so much that I will never forget it for the MCAT though....

 

[FrkyBgStok]

I tried the EK method, had it down pat and got to the torque section in the 1001 series and was completely lost. The best way that works for me is to choose a side. The distance to the fulcrum is x. So the other distance would naturally be 1-x. That is your lever arms. If the board is massless, awesome. If not, add the F(l) to the side the COM is and the lever arm is the distance to the COM. I know this sounds confusing and there is more algebra, but that EK method messed me up.

 

[funklab]

I tried the EK method, had it down pat and got to the torque section in the 1001 series and was completely lost. The best way that works for me is to choose a side. The distance to the fulcrum is x. So the other distance would naturally be 1-x. That is your lever arms. If the board is massless, awesome. If not, add the F(l) to the side the COM is and the lever arm is the distance to the COM. I know this sounds confusing and there is more algebra, but that EK method messed me up.

Wait... so you're saying the EK strategy led you astray in the EK 1001 problems!?!?!? If it doesn't even solve their own problems, what is it good for?

 

[FrkyBgStok]

Haha. I agree, that's why I don't use it.

 

[MedPR]

So Exam Krackers seems to abstractly choose a point of rotation that is not really the point of rotation at all in order to "simplify" the problem. I really have trouble wrapping my mind around their method.

It just seems incredibly counter-intuitive to choose a point of rotation that isn't one.

Is there an easier way, or is the EK method really the best/fastest way to solve an MCAT torque problem?

Move your force vector if possible so that theta = 90. That's the only shortcut I can think of.

 

[milski]

I don't have EK, so that's mostly a guess about what they're trying to say. It is true that when there is a rotation, there is a single rotation center* that you cannot change. But if you are considering a problem related to static stability, where you have a plank, a fulcrum, strings, weights etc which all are in equilibrium, you know that there is no rotation. No rotation means that there is not rotation around _any_ point in the system, so the torque at any point is zero. You can pick any point for your torque calculation and you should end up with a zero. A typical simplification in that case is to pick a rotation around a point where one of the forces is applied. Since the arm for that force is zero, you have one less torque to calculate.

*There can be a moving a moving center of rotation, but that's not something that would be considered in intro physics.

 

[MedPR]

So Exam Krackers seems to abstractly choose a point of rotation that is not really the point of rotation at all in order to "simplify" the problem. I really have trouble wrapping my mind around their method.

It just seems incredibly counter-intuitive to choose a point of rotation that isn't one.

Is there an easier way, or is the EK method really the best/fastest way to solve an MCAT torque problem?

Also, if the question(s) is asking you for certain forces in the system, "moving" the point of rotation will make finding certain forces much easier. For instance, changing the rotation point can allow you to find forces based on Torque net = 0.

 

[Mountaineurons]

So true. I have never felt so disconnected from Jordan and Jon - it's the first time in using their materials I'm seriously struggling.

 

[samser]

Seriously, I'm kind of glad that other people had problems with this I was SCREAMING last night at how terribly I was doing!