Balancing torque

[MrNeuro]

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http://i.imgur.com/a0Q97.jpg

EK says that

if you set T1 as your fulcrum
"the tension T2 pulling up at 2.5 m to the right of the pivot would also tend to rotate the board clockwise"

Can someone explain to me why this isn't counter clockwise???????? It's pulling up on the right side of the fulcrum how the hell is that rotating it clockwise that's counterclockwise!!! What am I doing wrong???

 

[milski]

The author probably has a digital watch. Or more likely, it's a typo. The torque from T2 around T1 is out of the page and the rotation is counterclockwise.

 

[MrNeuro]

The author probably has a digital watch. Or more likely, it's a typo. The torque from T2 around T1 is out of the page and the rotation is counterclockwise.

right thats what i thought but then they solved it another way w/ their weird sign convention but essentially they get the same thing that i would get except reversed and get an answer thats different if you solve it by setting t1 as the fulcrum and looking at the torques (way i asked about above)

they used
Fnet = 0
t1 + t2 = 10g + 20g

t2 = 30g - t1

torque net = 0

if T2 is fulcrum
clockwise
2.5(T1)
counterclockwise
10(5)g (10 kg mass on left side)

thus T1=200 N
T2= 100N

Where as the way i initially approached find

fulcrum T1
clockwise
20(2.5)g (the 20 kg board)
counterclockwise
10(2.5)g (10 kg mass)
T2(2.5) (self explanatory)

50g = 25g + 2.5T
25g = 2.5T
T= 10g = 100 N......IIIDDDIIIOOOT LOOOL NVM I GOT THE SAME ANSWER GAHHHH

 

[pfaction]

Is the answer 100? Probably not.

My math setting T1 as fulcrum:

Clockwise:
10(10)(2.5) = T2(2.5) + 20(10)(2.5)
250 = 2.5*T2*500
-250/2.5 = T2
-100 = T2 (counterclockwise obviously)

 

[milski]

So what do they set as fulcrum? In post 1 it's T1 but in post 2 it's T2??

 

[MrNeuro]

So what do they set as fulcrum? In post 1 it's T1 but in post 2 it's T2??

The ek 1001 book uses t2 their website posts an alternate/faster solution where they set it as t1

i guess i learned that you can always find the tension values via Fnet

 

[pfaction]

You had it right the first time; I initially got 50 because I forgot to multiply the mass of the board by 10, so my answer came out to 100 = T2+50 (20*2.5 vs 200*2.5 makes a huge difference here!)

 

[MrNeuro]

You had it right the first time; I initially got 50 because I forgot to multiply the mass of the board by 10, so my answer came out to 100 = T2+50 (20*2.5 vs 200*2.5 makes a huge difference here!)

yea looking back at my work i forgot to multiply T2 by lever arm

thanks milski and pfaction for a second there i was questioning everything i knew about clocks lol