Right hand rule-necessary?

[MShopes]

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The official MCAT guide outline does not show any right hand rule at all and I never see it in any AAMC test. Any one had it in real MCAT?

 

[cham90]

I had one on the 7/16 MCAT, although it could have been an experimental question. It's not too bad, here's how I do it.

Take your right hand and point your 4 fingers towards the first vector. Then bend your fingers towards the second vector (bend them in the way they normally bend, not a crazy contortionist configuration). The direction your thumb points is the direction of the cross product resultant vector. It will be perpendicular to both vectors, and its magnitude is the length of the two vectors multiplied by the sine of the angle between them (unlikely to be tested).

For example, East cross North is up (out of the map if you're looking down on one). West cross North is down (into the ground/map).

For North cross South the resulting product has a magnitude of zero, since these two vectors are parallel (sine of the angle between them = 0).

 

[PingPongPro]

There are many topics that are not explicitly stated on the MCAT official guide that we are supposed to know.

 

[Kimchii]

If you're lucky it wont be on it. I'd know it anyways just to be on the safe side.

 

[PingPongPro]

Oh, when I read the OP i thought he was asking about the right-hand rule regarding magnetic fields. I think this is a lot more likely to be asked than the right-hand rule to solve for cross product.

I think both are pretty easy to learn, so it wouldn't hurt to learn them. I don't think the test will ever specifically ask questions on the right hand rule per say(i.e. which direction does your finger point), but rather a topic on physics that can be answered using the right-hand rule or with physical intuition.

 

[MShopes]

Thanks guys, the right hand rule isn't hard to learn. I just wanted to make sure because I keep forgetting the directions so I guess I will have to look at my textbook diagram for it again.

 

[mitchlucker]

Yeah, know it..look it up...you never know...

 

[cham90]

Oh, when I read the OP i thought he was asking about the right-hand rule regarding magnetic fields. I think this is a lot more likely to be asked than the right-hand rule to solve for cross product.

I think both are pretty easy to learn, so it wouldn't hurt to learn them. I don't think the test will ever specifically ask questions on the right hand rule per say(i.e. which direction does your finger point), but rather a topic on physics that can be answered using the right-hand rule or with physical intuition.

They should be one and the same. See: http://en.wikipedia.org/wiki/Magnetic_field#Force_on_moving_charges_and_current

When you memorize F=q*v*sin(theta) and use the right hand rule for the direction you've simply substituted in the definition of the cross product 🙂 I agree if you're saying this specific equation is where it would most appear on the MCAT.

 

[MShopes]

I had one on the 7/16 MCAT, although it could have been an experimental question. It's not too bad, here's how I do it.

Take your right hand and point your 4 fingers towards the first vector. Then bend your fingers towards the second vector (bend them in the way they normally bend, not a crazy contortionist configuration). The direction your thumb points is the direction of the cross product resultant vector. It will be perpendicular to both vectors, and its magnitude is the length of the two vectors multiplied by the sine of the angle between them (unlikely to be tested).

For example, East cross North is up (out of the map if you're looking down on one). West cross North is down (into the ground/map).

For North cross South the resulting product has a magnitude of zero, since these two vectors are parallel (sine of the angle between them = 0).

Shouldn't it be the opposite? It actually depends if you use the inner of your hand for the right hand rule vs using the out of your hand. From my textbook and online, it seems like we use the inner part of our hand (palm and fingers from inside) to do the right hand rule. Using this, the fingers will be pointing toward north first (up in the map or in the direction going away from us) and when I bend them, they will point to the left (west). With that, the thump is going to be up which is the cross product. For the east part, you simply invert your hand to be upside down to still use your inner hand and the fingers again will be pointing toward north first and then when I bend them, they will point to the right (east) and the thump will point down this way.

I can't imagine a cross product of north vs south since they are parallel vectors going into opposite directions.

 

[centrigeugle]

No, he's right.

And I also had right-hand-rule problems on my MCAT.

 

[whiteshadodw]

While the official AAMC topics list is the best source for what you need to know, its not the end-all-be-all list. Seriously, if you've taken practice tests you'll know what I mean. There are definitely some questions which aren't on the topics list which you are just assumed to know. That's why a 45 is so hard to attain, there will definitely be one question that you either knew or didn't. Right hand rule is one of those things that's covered in pretty significant detail in all intro physics classes. I would definitely know it.