Determining mass of balance bar

  • Thread starter izmeh
  • Start date
  • #1
Here's the situation proposed in a problem.
I have a balance bar that is a meter stick.
It has a fulcrum at 50cms. Mass A(100gms[.98nt]) is at 12.3cms and mass B(1.02nt) is located at 86.1cms.
Mass B has been removed and the fulcrum C shifts fo the left by 14cms.
I need to determine the mass of the meter stick. How do i do this?
 

Answers and Replies

  • #2
HallsofIvy
Science Advisor
Homework Helper
43,021
970
The first part "It has a fulcrum at 50cms. Mass A(100gms[.98nt]) is at 12.3cms and mass B(1.02nt) is located at 86.1cms." just tells you that the bar itself has center of mass located at its center (since A is 50-12.3= 37.7 cm from the center and B is 86.1-50= 36.1 cm from the center- 37.7*.98= 36.1*1.02 (well, close enough for experimental error)).

After B is moved, the fulcrum shifts "14 cm to the left" so it is now at 36 cm. We still have mass A at 12.3 cm, 23.7 cm from the fulcrum. Its torque is .98*23.7= 23.226 (actually 0.23226 N-m). The center of mass of the bar is still at 50, 14 cm from the fulcrum so, taking the weight of the bar to be W, the torque around the fulcrum is
14W. To be balanced we must have 14W= 23.226 or W= 1.659 N. That means that the mass of the bar is 1.659/9.81= 0.169 kg or 169 grams.
 
  • #3
thank you
 

Suggested for: Determining mass of balance bar

  • Last Post
Replies
4
Views
2K
Replies
4
Views
473
  • Last Post
Replies
3
Views
603
Replies
13
Views
422
  • Last Post
Replies
7
Views
2K
Replies
2
Views
400
  • Last Post
Replies
7
Views
1K
  • Last Post
Replies
5
Views
8K
  • Last Post
Replies
6
Views
10K
Top