Rate of rotation and inertia (prevent homicide and help me, please)

In summary, the conversation centers around a difficult problem on an online physics quiz. The problem involves a diver rotating at 1 revolution per second in a stretched out position and then tucking her head in and bending her legs, shortening her length by 1/2. The question is asking for her rate of rotation in this position, with the choices of 4 Hz or 5 Hz. The conversation includes attempts at solving the problem, including using the concepts of rotational inertia and angular momentum. The conversation also expresses frustration with online physics quizzes and the overall online class format.
  • #1
NoHeart
28
0
i understand 4 of the 6 problems on this week's online physics quiz, but this one is driving me completely NUTS!

a diver rotates at 1 revolution per second in the stretched out position. when the diver tucks her head in and bends her legs, assume her length is shortened by 1/2. what is her rate of rotation in this position?
my choices are 4 Hz or 5 Hz, you'd think having only 2 choices would make this easier, but alas, i am dumbfounded.

i have tried thinking of the diver in the stretched out position as a rod, with rotational inertia 1/12ML^2
the other position would be like a solid sphere, with rotational inertia 2/5MR^2

1 revolution per second is 1 Hz, or 2pi r/s

the angular momentum (which may be irrelevant) of the stretched out position is
L= 1/12M(2r)^2 * 6.28 radians/1 revolution * 1 revolution/1second
this leaves me with L=2.093Mr^2 rad/sec

L of ball position is
2/5Mr^2 * 6.28 radians/?sec= 2.51Mr^2 radians/?sec
?=0.39Mr^2 rad/sec

slowly going insane, i see that rotational accleration = net torque/rotational inertia
this also leads me nowhere

any help, hints, or a slap across the face would be greatly appreciated

(p.s.- anyone else think online physics is the STUPIDEST idea EVER?)
 
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  • #2
*Slaps NoHeart in the face*, "wake up man!"

Lol...just kidding. :tongue2:

Keep in mind these quizzes never have super complex questions, they are always just testing the basic concepts. In this case, angular momentum, L, can be expressed as:

[tex] L = I\omega [/tex]

Where I is the moment of inertia. Keep it simple, model the diver as having the same overall shape, whether she's fully extended or not. After all, they only give you length, [itex] l [/itex], to work with. So if:

[tex] I \propto l^2 [/tex]

then when [itex] l [/itex] is halved, by what factor does I decrease? _________

But if I decreases by that factor, and by conservation of angular momentum, the product [itex] L = I\omega [/itex] must remain constant, then by what factor does [itex] \omega [/itex] increase to compensate?

Yes, I also feel that online physics quizzes are a stupid idea.
 
  • #3
1/4! i love you!
and i wish it were only the quizzes that are online, but it's the whole class- the online "lectures" are simply outlines of the chapters in the book, and the book is completely useless when it comes to the quizzes and tests. i have been teaching myself all of the concepts involved in the class so far, with the help of many physics sites and this here amazing message board. if i had found this board at the beginning of the quarter, i'd probably have an A instead of a B.
 

1. What is the rate of rotation and how does it relate to preventing homicide?

The rate of rotation refers to the speed at which an object rotates around an axis. It is important in preventing homicide because it affects the amount of force that can be applied by an object in motion. The faster an object rotates, the more force it can exert, making it more dangerous in the case of a potential homicide.

2. How is inertia connected to preventing homicide?

Inertia is the property of an object to resist changes in its motion. In the context of preventing homicide, inertia plays a role in reducing the impact force of an object. This means that an object with a high inertia will be harder to stop or slow down, making it less likely to cause serious harm or death in a potential homicide situation.

3. Can understanding the principles of rate of rotation and inertia help me defend myself in a dangerous situation?

Yes, understanding these principles can help you defend yourself in a dangerous situation. By knowing the rate of rotation and inertia of objects around you, you can better assess the potential danger and take appropriate actions to protect yourself.

4. How can I improve my understanding of rate of rotation and inertia in order to prevent homicide?

You can improve your understanding by studying physics and mechanics, as these concepts are fundamental principles in these fields. You can also consult with experts in these fields or attend workshops and seminars to learn more about how these concepts relate to preventing homicide.

5. Are there any real-life examples of how understanding rate of rotation and inertia has helped prevent homicides?

Yes, there have been cases where understanding these principles has helped prevent homicides. For example, in self-defense situations, individuals have been able to use their knowledge of the rate of rotation and inertia of objects around them to defend themselves and neutralize potential threats without causing serious harm to the attacker.

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