How can I understand Rotational motion and dynamics better?

[misskitty]

I need some study help with Rotational motion and dynamics. I have to teach myself the material because I can't go to school. I need help learning how to apply the material and how to remember the equations. Can anyone give me any suggestions and study help tips? Anything you can provide would be greatly appreciated.

MissKitty

 

[scholzie]

The best way to study physics is by doing many, many, many problems until you have a good idea about how to tackle a situation. Remember that physics isn't about solving big problems; it's about breaking big problems into small problems and solving them all, one at a time. As with any other thing, physics gets easier with experience, and you will learn the best way to deal with situations. It doesn't seep in without the work, though!

The internet is a wealth of information regarding practice problems (many universities use online material, where a professor may have posted practice problems for his or her students). I suggest "googling" for whatever topic you need, and adding the words "practice problems" after it.

Rotational motion is fairly straight forward in that you have very few equations to memorize, and they're pretty complementary to things you know already.

Was there anything specifically that you needed help with?

 

[misskitty]

Not really anything specific. Some of it is specific like: How do you know where to put your lever arm in a problem that asks you to calculate how much torque is being applied to the object in equilibrium, with several other forces involved? I just need some guidance on how to break the stuff down.

When I have been able to attend school, I've sought help from my physics teacher and the engineering teacher. Both have helped but I still don't entirely comprehend how to break it down and what to do and what to use and stuff.

Its frustrating since I've read the chapter about 4-5 times and tried to memorize and apply the questions. I just get lost. Is there any way to understand it all.

Any other suggestions?

 

[scholzie]

Not really anything specific. Some of it is specific like: How do you know where to put your lever arm in a problem that asks you to calculate how much torque is being applied to the object in equilibrium, with several other forces involved? I just need some guidance on how to break the stuff down.

The first step is always always always draw a Free-body diagram with all of the forces involved. That means gravity, normal force, and all of the forces applying torques. You know that if the object is in equilibrium, then ALL of the forces are canceled out by their opposing ones, and all of the TORQUES are also canceled out. This drawing is important even on the "simple" problems, and even experienced people use them. Don't be ashamed to draw a picture!

The key in this particular situation is to find all the torques acting clockwise, and all of the torques acting counter-clockwise, and set them equal to each other (Or, subtract one set of torques from the other, and set them equal to 0). You may very well not know the size of some of the torques, so you must use symbols instead of numbers. That's ok. If there is enough information in the problem, it will all work out eventually.

The next thing to realize is that the sum of the downward forces will equal the sum of the upward forces, and the sum of the leftward forces will equal the sum of the rightward forces ($F_y=\sum F_{up} + \sum F_{down} = 0,\ and \ F_x=\sum F_{left}+\sum F_{right} =0$). Since all of the forces cancel, the object remains at rest. If you are unfamiliar with the $\sum$ character, it simply means "sum".

After that, it's a simple matter of solving for whatever unknown variable you have, and you're done! It's always best to take any problem and try and picture what's going on. Always draw a picture, and always label the forces. If you know something about the situation, you should then be able to add up the forces and set them equal to something (0 if equilibrium, ma if moving)...then work backwards from there.

Is there any way to understand it all.

Sadly, no, although some of us would like to think we know it all