Physics help!!

[iamwiz82]

ahhh!! i have a test in 2 days on rotation of a rigid body about a fixed axis and i have idea what i am doing.

I have tried asking the prof, but i come out of the talk understanding less. Can anyone point me to a good site for learning this stuff?

here's a sample problem:

A pulley consists of 2 disks of different diameters attached to the same shaft. The rope connected to the block of a mass M1=1kg passes over a smooth peg, while the block of mass M2=3kg hangs vertically from one disk. The moment of Inertia of the pulley is 0.2 kg*m^2; R1= 5cm, and R2=10cm. Find the tensions in the ropes, and the acceleration of the blocks.

yikes...

 

[AnimeKnight]

I thought you are a wiz
sorry I can't help you... physics bring back the bad memories

 

[scorp00]

That's the hard sh!t. Thank god my professor believed in multiple choice, only way I passed physics.

 

[iamwiz82]

Anime, notice it is wiz, its a pun on my last name, which begins the pronunciation "Wis." hehe

Anyways, my tests are multiple choice, but you get no credit if you dont show all the work. sigh...

Physics geniuses help!

 

[Ameesh]

explain to me what exactly your having problems with. i dont want to tell you everything cause half of this stuff is about really figuring out but i dont mind pointing out some promising avenues

 

[iamwiz82]

well, i dont understand the actual circular motion

I know that T-M1G=M1A, and T=M2A+M2G,, i could find it out if it wasnt for the pulley.

 

[Jothaxe]

do you want a detailed solution for the problem you posed? or just general info?


Pulleys arent different than any other object, just remember:

1) if static, then net forces and net torques must be zero

2) if in motion, F.net = m*a , and Torque.net = (moment of inertia)*(angular acceleration)


can anyone verify if the torque equation is right? I havent done this for years, but it looks right

 

[Ameesh]

so you get confused when you put the pully into the problem?

dont let that part screw with you, just imagine that the pully is an easy way to redirect the forces. if i were you i would start by finding the acceleation of the blocks. the only problem is that everything uses angualr acceleration but remeber they are the same concept as the normal acceleration so angualr accl. is == delta (angualr velocity)/ delta time. just take these problems one step at a time. and make sure you understand what each part of the formulas are for.

 

[Ameesh]

torque = radius^2 * mass * angualar acceleration == t = mr^2a

mr^2 is the rotational inertia and is called the moement of inertia

 

[Ameesh]

oops,, Jothaxe is right!

 

[Ameesh]

(oops ment i didnt say verified, im right too, we are both right !, actually we had the same exact answer)

 

[rmeijer]

The use of a good "Free Body Diagram" is essential in figuring this stuff out.

I'd recommend sketching it out the whole system with all the forces penned in for good measure.
Write down all the equations that you think are relevant, then try to put it all together (this last step takes some practice... do a search on Free Body Diagrams to see what you come up with).

If you haven't learned about FBD, better start....

 

[iamwiz82]

rmiejer, i know free body diagrams. My problem has been with 1/2Iw^2, theta=theta(naught)+w(naught)*t+.5(angular acceleration)(t)^2, etc. Its the new formulae using old notation. My mind is stuck in linear kinematics, i think.

I did figure out that tension=(moment of inertia*angular accel.)/radius

this really has cleared things up.

 

[Jothaxe]

<< actually we had the same exact answer >>



What is it they say about great minds, Ameesh? Hehe

 

[The Wildcard]

Ahh mechanics!!!!