ExamKrackers really does a poor job of addressing circular motion

[atomi]

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[Mandek]

Although EK books are great review books, I wouldnt rely on them solely for the MCAT. Study from additional materials.

 

[Mandek]

So far, I have not been impressed with the EK books. They are full of an ungodly amount of typos (seriously, this book is printed like a 1st draft -- did they even have an editor?), and information like this seems to be blatantly omitted. I really worry what kind of information I am learning from subjects that I don't know as well as physics.

Yeah, they have a lot of mistakes. Even in 1H simulated exam that I got, they have 4 questions numbered 88 instead of 88, 89, 90, 91 etc. Its so annoying sometimes!

 

[xrevision]

Taken from wikipedia: "Centripetal force should not be confused with centrifugal force. The centrifugal force is a fictitious force that arises from being in a rotating reference frame. To eliminate all such fictitious forces, one needs to be in a non-accelerating reference frame, i.e., in an inertial reference frame. Only then can one safely use Newton's laws of motion, such as F = ma."

So in a non rotating reference frame, the force you get is the one that is keeping the object along the circular path - centripetal.

 

[unsung]

Particularly, it makes no mention of the difference between centripetal force and centrifugal force. This is critical because by just reading the EK Physics book, you are left with the impression that objects in a circular path are pulled inwards, which is completely false. They are pulled outwards (think of the centrifuge amusement park ride). Furthermore, the book goes on to ask (many) questions in its lecture exams that require centrifugal understanding without ever discussing it!!!

I highly recommend that everyone read the wikipedia articles on centrifugal force and centripetal force and fully understand each.

So far, I have not been impressed with the EK books. They are full of an ungodly amount of typos (seriously, this book is printed like a 1st draft -- did they even have an editor?), and information like this seems to be blatantly omitted. I really worry what kind of information I am learning from subjects that I don't know as well as physics.

Uh... "centrifugal force" does NOT exist. Centripetal force is the real force in play when you tie a mass to a string, and swing the mass around in a circle. The only force on the weight (aside from gravity) is the tension of the rope, pulling the weight inwards: centripetal force.

Yes, the mass in turn, reacts by pulling on the string with a force directly opposite to the direction of the tension. But this is irrelevant. Again, just considering the forces acting ON the mass, there is no force directed outwards of the circular path. It "seems" like there is a force directed outward, but that's just because of inertia- objects in motion (in this case, the mass wants to continue going in a straight line) tend to resist changes in their motion. But again, there is no actual force pulling the mass outwards of its circular path.

 

[RSAgator]

Particularly, it makes no mention of the difference between centripetal force and centrifugal force. This is critical because by just reading the EK Physics book, you are left with the impression that objects in a circular path are pulled inwards, which is completely false. They are pulled outwards (think of the centrifuge amusement park ride). Furthermore, the book goes on to ask (many) questions in its lecture exams that require centrifugal understanding without ever discussing it!!!

I highly recommend that everyone read the wikipedia articles on centrifugal force and centripetal force and fully understand each.

So far, I have not been impressed with the EK books. They are full of an ungodly amount of typos (seriously, this book is printed like a 1st draft -- did they even have an editor?), and information like this seems to be blatantly omitted. I really worry what kind of information I am learning from subjects that I don't know as well as physics.

I think EK said pretty clearly that while objects are pulled inwards, they maintain a circular path because the direction of the acceleration is constantly changing causing it to spin in a circle with a constant velocity rather than actually moving inward. This, along with the equations, are really all there is to know about centripetal force (for the sake of the MCAT at least).

I don't really see what was "blatantly omitted." If there are other things that were (which you seem to imply) then please let us know. Just check the AAMC topic list for physical sciences first

http://www.aamc.org/students/mcat/topics.pdf

 

[canjosh]

I think EK said pretty clearly that while objects are pulled inwards, they maintain a circular path because the direction of the acceleration is constantly changing causing it to spin in a circle with a constant velocity rather than actually moving inward.

http://www.aamc.org/students/mcat/topics.pdf

I believe that the velocity in circular motion is constantly changing...the path is circular and therefore the direction of the object is constantly changing. The speed is constant. The acceleration is always pointing to the center as others have explained. Somebody correct me if I'm wrong in saying that the velocity is not constant...

 

[xrevision]

I believe that the velocity in circular motion is constantly changing...the path is circular and therefore the direction of the object is constantly changing. The speed is constant. The acceleration is always pointing to the center as others have explained. Somebody correct me if I'm wrong in saying that the velocity is not constant...

You are correct sir.

 

[ChubbyChaser]

EK was piss poor in general, I regret my decision.

 

[RSAgator]

You are correct sir.

indeed...meant magnitude of the velocity

 

[atomi]

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[xrevision]

Quoted from the 6th Edition of "Physics, Principles with Applications" by Giancoli, published by Pearson Education, Inc. (so not just some random people putting stuff up on wiki):

"There is a common misconception that an object moving in a circle has an outward force acting on it, so-called centrifugal ("center-fleeing) force. This is incorrect: there is no outward force on the revolving object. consider, for example a person swinging a ball on the end of a string around her head. If you have ever done this yourself, you know that you feel a force pulling outward on your hand. The misconception arises when this pull is interpreted as an outward "centrifugal" force pulling on the ball that is transmitted along the string to your hand. This is not what is happening at all. To keep the ball moving in a circle, you pull inwardly on the string, and the string exerts the force on the ball. The ball exerts an equal and opposite force on the string (Newton's third law), and this is the outward force your hand feels.

The force on the ball is the one exerted inwardly on it by you, via the string. To see even more convincing evidence that a "centrifugal force" does not act on a ball, consider what happens when you let go of the string. If centrifugal force were acting, the ball would fly outward. But it doesn't; the ball flies tangentially in the direction of the velocity it had at the moment it was released, because the inward force no longer acts."