# Quick Physics torque question - Need a check

• Jan 4th 2009, 02:50 PM
Intrusion
Quick Physics torque question - Need a check
So I've gotten an answer, but my percent error was huge, which is why I want to make sure that I did it right. Can someone do this real quick and tell me what they get?

http://i42.tinypic.com/dxk9wl.png
[not drawn to scale]

Thanks!
• Jan 4th 2009, 03:03 PM
skeeter
net torque caused by the 10 gram mass ?

(.010)(9.8)(.48) = .047 N-m

I get x = 2.4 cm to balance the system using the 200 gram mass.
• Jan 4th 2009, 03:07 PM
Intrusion
Thanks, that's what I got. I guess it's meant to have a huge percent error. What could be the cause of a 70% error? (the "actual" value should have been 8m with a 200 kg mass)
• Jan 4th 2009, 03:08 PM
Mush
Quote:

Originally Posted by Intrusion
So I've gotten an answer, but my percent error was huge, which is why I want to make sure that I did it right. Can someone do this real quick and tell me what they get?

http://i42.tinypic.com/dxk9wl.png
[not drawn to scale]

Thanks!

$M_1 = F_1 \times d_1 = 0.48m \times 0.01(9.81) = 0.047Nm$

$\Sigma M = M_1+M_2 = 0.047-(x\times 0.2 \times 9.81) = 0$

Hence:

$x = \frac{0.047}{0.2\times 9.81} = 0.024m$
• Jan 4th 2009, 03:08 PM
RossBrons
Since 200 is 10 x 20

It stands to reason that 48/20 = x

Thus 2.4 cm

EDIT ---------------------------------

• Jan 4th 2009, 03:11 PM
Mush
Quote:

Originally Posted by Intrusion
Thanks, that's what I got. I guess it's meant to have a huge percent error. What could be the cause of a 70% error? (the "actual" value should have been 8m with a 200 kg mass)

But it isn't a 200kg mass, it's a 200g mass. 8m is ridiculous compared to the tiny torque exerted by the 10g mass.
• Jan 4th 2009, 03:13 PM
Intrusion
Quote:

Originally Posted by Mush
But it isn't a 200kg mass, it's a 200g mass. 8m is ridiculous compared to the tiny torque exerted by the 10g mass.

Yeah, I meant a 200g mass, not kg.
• Jan 4th 2009, 04:33 PM
Mush
Quote:

Originally Posted by Intrusion
Yeah, I meant a 200g mass, not kg.

Even at that, 8m would produce far too much torque.

10g mass is placed 48cm away, to balance this a 200g mass would be a lot closer than 48cm... and as you know 8m is far from being closer than 48cm.
• Jan 4th 2009, 05:24 PM
Intrusion
Alright, I'll have to enlighten my teacher then and let him know his "actual" answer is wrong. Thanks!