Torque - Where is the normal force in this FBD?

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Homework Help Overview

The discussion revolves around a physics problem involving torque and forces acting on a metre-stick with a bug crawling on it. The original poster seeks clarification on the location of the normal force in a free body diagram (FBD) as the bug moves along the stick, which is partially overhanging a desk.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • The original poster questions the placement of the normal force, considering its position relative to the stick's center of mass and the edge of the desk.
  • Some participants suggest that the normal force should be located at the center of mass, while others argue it acts at the point of contact with the desk.
  • There is exploration of how the bug's position affects the torque and stability of the stick.
  • One participant raises the issue of the overhang distance and its impact on the bug's movement without tipping the stick.

Discussion Status

The discussion is ongoing, with participants sharing different perspectives on the normal force's location and its implications for the problem. Some guidance has been offered regarding the relationship between the normal force and the center of mass, but no consensus has been reached on the correct interpretation.

Contextual Notes

Participants note the importance of the overhang distance of the metre-stick on the desk, which is specified as 0.70m in a later post. This detail is crucial for understanding the dynamics of the system.

Whotto
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Silly question I know, but I want to just clarify this.

The actual question:
A 0.45kg bug is crawling to the right along a metre-stick which is resting on the edge of a desk. If the mass of the uniform metre-stick is 0.37kg, how far from the edge of the desk can the bug crawl before the metre-stick flips off the desk?

[PLAIN]http://img842.imageshack.us/img842/9154/58811064.jpg

My question:
Eventually, I worked out that FN is the green arrow. But why is it there instead of being at where the red arrow is? I realize the entire table is the support. So should it not be in the middle of where the support touches the metre-stick?

I also know found no way to solve this without putting FN in the middle. I got the answer, but I want to clarify this and double-check.

[PLAIN]http://img194.imageshack.us/img194/9067/24168499.jpg

In case I am not correct by putting FN in the middle, the answer I got is 0.1644m. Is that right?

Many thanks!
 
Last edited by a moderator:
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the normal force should be where the center of mass is.
 
Hi MHrtz, thanks for the reply.

Do you mean this then?

[PLAIN]http://img153.imageshack.us/img153/2333/95048564.jpg

I can't solve the question if it is this, because any weight on the right side will tip the ruler - which isn't suppose to happen.
 
Last edited by a moderator:
Okay, I after further pondering, I realized that FN could range from mg to 0. So that means the bug could only have as much torque as mg.

So the answer would be 0.16m. Am I right or still totally wrong?
 
Last edited:
You don't indicate how far from the edge of the table that the meter stick is projecting. If initially, with the bug at the edge of the table, the stick was delicately balanced half on the table, and half off, the bug could not move an iota of a mm from the edge of the table, without tipping the stick over. What's the overhang distance?
 
My apologies...I forgot to include that 0.70m of the stick is in contact with the table. Sorry! I am not missing any other information though, I double-checked to make sure.
 
Whotto said:
Okay, I after further pondering, I realized that FN could range from mg to 0. So that means the bug could only have as much torque as mg.

So the answer would be 0.16m. Am I right or still totally wrong?
You are right. But the normal force is always equal to the stick's plus bugs weight...its resultant location varies as the bug moves to the right, ultimately ending up as a concentrated force at the at the edge of the table at the point of pending tipover.
 
PhanthomJay said:
You are right. But the normal force is always equal to the stick's plus bugs weight...its resultant location varies as the bug moves to the right, ultimately ending up as a concentrated force at the at the edge of the table at the point of pending tipover.

Ah! I get it now. Thank you very much :)
 
normal always acts through the center of mass
ie the center of the rod in this case
 
  • #10
Selwyn said:
normal always acts through the center of mass
ie the center of the rod in this case
No. The resultant weight always acts through the center of mass...the normal (perpendicular contact force) does not always act through the center of mass.
 
  • #11
PhanthomJay said:
No. The resultant weight always acts through the center of mass...the normal (perpendicular contact force) does not always act through the center of mass.
hm...you are right
normal acts at the point of contact
 

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