FlyVirgin744
Topic Author
Posts: 1282
Joined: Tue Jul 27, 1999 8:35 am

Need Help: Physics!

Fri Mar 07, 2003 3:22 am

Hey guys, I've been stuck on this problem for awhile. Maybe someone can help me.

Figure 11-43 shows two blocks, each of mass m = 3.2 kg, suspended from the ends of a rigid massless rod of length L1 + L2, with L1 = 0.20 m and L2 = 1.8 m. The rod is held horizontally on the fulcrum and then released. What are the magnitudes of the initial accelerations of (a) the block closer to the fulcrum and (b) the other block?

Now the answer to this situation is

0.956 m/s^2 and 8.60 m/s^2. But next time I do it I will get new numbers.

Can anyone help?
Sometimes I go about in pity for myself and all the while a great wind carries me across the sky.
 
airways1
Posts: 536
Joined: Mon Jul 19, 1999 3:05 am

RE: Need Help: Physics!

Fri Mar 07, 2003 4:13 am

I've just done a quick calculation, and I think the answers are

(a) 0.16 m/s^2

(b) 1.44 m/s^2

I will post again if I come up with a different answer.

airways1
 
JetService
Posts: 4611
Joined: Mon Feb 21, 2000 1:12 pm

RE: Need Help: Physics!

Fri Mar 07, 2003 4:15 am

Ask David B. He's an expert on massless rods.
"Shaddap you!"
 
Staffan
Posts: 3879
Joined: Tue Mar 18, 2008 9:21 am

RE: Need Help: Physics!

Fri Mar 07, 2003 4:19 am

Hi,

First calculate the torque around the fulcrum.

T=L2*F2-L1*F1

Then use Newton's second law for rotation

T=I*a/r (r=L in this case) where I=I1+I2 and I1 and I2 are the moment of inertia for each of the two blocks added together (I=mr^2)

This gives a1=(T*L1)/I and a2=(T*L2)/I

I got the answers you posted above doing it this way. Hope it helps!

Staffan

 
airways1
Posts: 536
Joined: Mon Jul 19, 1999 3:05 am

RE: Need Help: Physics!

Fri Mar 07, 2003 4:25 am

I seem to have the same problem as you - I get a different answer each time.

Now I have (taking g=10N/kg)

(a) 0.99 m/s^2

(b) 8.89 m/s^2

I thought my previous answer seemed a bit low...

airways1
 
FlyVirgin744
Topic Author
Posts: 1282
Joined: Tue Jul 27, 1999 8:35 am

RE: Need Help: Physics!

Fri Mar 07, 2003 4:57 am

That was it Staffan, thanks a lot  Smile
Sometimes I go about in pity for myself and all the while a great wind carries me across the sky.
 
Toner
Posts: 247
Joined: Sat Feb 15, 2003 11:53 pm

RE: Need Help: Physics!

Fri Mar 07, 2003 5:00 am

http://panda.unm.edu/courses/finley/P160/HW/HW31Sol.html
 
flight152
Posts: 3211
Joined: Fri Nov 24, 2000 8:04 am

RE: Need Help: Physics!

Fri Mar 07, 2003 5:17 am

JetService..your a math expert. Can you ramble off some of your bullshit errr.. math knowledge?
 
JetService
Posts: 4611
Joined: Mon Feb 21, 2000 1:12 pm

RE: Need Help: Physics!

Fri Mar 07, 2003 5:37 am

Flight152, I was going to see if someone else could help first. I hate to be a know-it-all, but after seeing the responses, I guess I have to.

FlyVirgin's first problem is his polyfractals are all askew. He doesn't realize (as most of the population, except me) that whenever you have two opposing unilinial lengths on a reflective plane, it immediately puts the fulcrum in an opposing mass state. This basically inverts your polyfractals incrementally, thus skewing them. To compensate when calculating the overall static basis, all you have to do is place your variables (L) in the same matrix as the subvortex. Otherwise you end up with a coefficient leak. And everyone know that a coefficient leak, while durinominal, is only a raw element in a saturated array. Hope that helps.
"Shaddap you!"
 
tbar220
Posts: 6706
Joined: Wed Feb 02, 2000 12:08 pm

RE: Need Help: Physics!

Fri Mar 07, 2003 9:13 am

ROFL JetService!!  Big thumbs up  Big thumbs up
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