Static equilibrium -- A picture hanging on a wall

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Homework Help Overview

The discussion revolves around a static equilibrium problem involving a uniform rod resting against a smooth wall and partially on a rough table. Participants are tasked with determining the net force exerted by the table on the rod, particularly focusing on the torque equations involved.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the setup of torque equations and the forces acting on the rod. There are questions about missing forces and the correct identification of torque arms. Some participants express confusion regarding the weight of the rod and its implications for the problem.

Discussion Status

There is an ongoing exploration of the forces involved, with some participants providing guidance on identifying the normal force and the weight of the rod. Multiple interpretations of the torque equations are being discussed, and participants are encouraged to break down the problem into smaller questions.

Contextual Notes

Participants note that the problem does not provide the weight of the rod explicitly, leading to discussions about assumptions regarding uniform mass distribution. There is also mention of unfamiliar concepts, such as cotangent, which some participants find challenging.

siapola1
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Homework Statement


https://imgur.com/v13K6sE
a uniform rod of mass m is placed as shown, with one of its end resting on a smooth wall while 1/4 of the rod's length is sticking out of a rough table. Find the net force the table exert on the rod at the corner.

Homework Equations


i drew the free body diagram and labeled all the forces. i get stuck when i tried to set up the torque equation
because i don't know what is the force that is responsible for rotating the rod.
i have the picture here:
https://imgur.com/ybSWfcT

3. The Attempt at a Solution
its in the picture
 
Last edited by a moderator:
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There are two things wrong with your torque equation.
1. There is a force missing.
2. Your expressions for the torques for the parts of the rod are wrong. For each part, what is its weight and what is its torque arm?
(Though you don’t really need to treat the rod as two parts. Just find the torque arm for the rod as a whole.)
 
siapola1 said:
he didnt give us the weight
You are told it is a uniform rod of mass m, so you can find the weight(s).
siapola1 said:
the missing force the normal force at the contact point between the table and the rod?
Yes.
Edit: I mean no... it is a normal force but not that one.
 
Last edited:
what is tikd?
 
siapola1 said:
what is tikd?
It is a result of fat fingers. Please see edited version.
 
i still can't figure it out

i check the answer and is this:
mg/3 [√(9+ cot2(φ))]

we have never encounter a problem that the answer is cotangent before its way out of my league can someone please just tell me how to do it?
 
i just checked the answer and it is
mg/3 ( √(9+cot2(φ))

we have never encounter a problem that its answer has cotangent in it. its way out of our league. can someone please just show me how to derive the answer?
 
siapola1 said:
can someone please just tell me how to do it?
That's not how this forum works. Don't be put off by the cotangent. Just try to answer these questions in sequence:
You are given the mass of the rod. What is the force of gravity on it?
Where does that force act?
What is the moment arm of that force about the point of contact?
Unfortunately I made a mistake in answering a previous post of yours. I'll correct that then continue...
 
  • #10
ok, I have edited post #4. You need to look for another force that has a torque about the point of contact between rod and table. The contact forces that act there cannot have a moment about it.
 

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