Why Is the Calculated Torque About the Shoulder Incorrect?

  • Thread starter Thread starter sona1177
  • Start date Start date
  • Tags Tags
    Torque
Click For Summary

Homework Help Overview

The discussion revolves around calculating the torque about the shoulder when an athlete holds a weight in their hand. The subject area includes concepts of torque, center of mass, and the application of physics principles in a biomechanical context.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • Participants discuss the calculation of torque, questioning the assumptions about the center of gravity and the angle involved. There is confusion regarding the correct distance to use for torque calculations and the implications of where mass is considered to be concentrated.

Discussion Status

The discussion is active, with participants providing insights into the misinterpretations of the problem setup. Some guidance has been offered regarding the correct application of distances and angles in torque calculations, but there is still some uncertainty about the center of mass and its relevance to the problem.

Contextual Notes

Participants are navigating through the implications of the problem statement, particularly regarding the definitions of distances from the shoulder and the assumptions about mass distribution in the arm and ball.

sona1177
Messages
171
Reaction score
1
An athlete at the gym holds a 3.0 kg ball in his hand. His arm is .70 cm long and has a mass of 4.0 kg. What is the magnitude of the torque about his arm if he holds his arm straight out his side, parallel to the floor. I did T=mgr since theta is zero so both the force of gravity on his arm and ball is down. So 7* 9.8*.70 where the mass is the mass of his arm + mass of the ball. I get 48 N-m but this is wrong the answer is 34 N-m. What am I doing wrong?
 
Physics news on Phys.org
This would by right if all mass of the arm were concentrated in the hand.

ehild
 
ehild said:
This would by right if all mass of the arm were concentrated in the hand.

ehild
So is .70 m not the center of gravity. I'm so confused ...
 
theta is not zero, it is 90 degrees. And 0.7 m is not the center of gravity. If you have a uniform stick, which length is 0.7 m and has a 4 kg mass, where is then its center of mass?
 
housemartin said:
theta is not zero, it is 90 degrees. And 0.7 m is not the center of gravity. If you have a uniform stick, which length is 0.7 m and has a 4 kg mass, where is then its center of mass?

.35 thanks :) so Now I add the torques but why do we consider the center of gravity for the ball to be .70 m?
 
because you need to calculate torque about the shoulder (well topic name says about shoulder, but in problem statement you say about the arm...). In order to do so, you must take distances from shoulder (its like shoulder is your coordinates starting point), and ball is 0.7 m away from the shoulder.
 
housemartin said:
because you need to calculate torque about the shoulder (well topic name says about shoulder, but in problem statement you say about the arm...). In order to do so, you must take distances from shoulder (its like shoulder is your coordinates starting point), and ball is 0.7 m away from the shoulder.

Thank you. I'm sorry, I should have said shoulder. Thank you again for your help.
 

Similar threads

Replies
1
Views
1K
  • · Replies 4 ·
Replies
4
Views
5K
  • · Replies 1 ·
Replies
1
Views
4K
  • · Replies 21 ·
Replies
21
Views
2K
  • · Replies 2 ·
Replies
2
Views
11K
Replies
6
Views
2K
  • · Replies 4 ·
Replies
4
Views
16K
  • · Replies 4 ·
Replies
4
Views
3K
  • · Replies 5 ·
Replies
5
Views
3K
  • · Replies 3 ·
Replies
3
Views
13K