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Direction of The Force of a Wall Exerted on a Rod

  • #1
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Homework Statement


A rod is attached to a wall in such a way it can swivel. In this case: In which direction does the force (of the wall on the rod) point to?
Here are two examples (see under attached files), but the the direction of the force is different. Why? Is maybe one of the pictures wrong?
In first example I drew the force with blue as I would draw force diagram.

Also: What is the recipe here? How do we determine the direction?
 

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Answers and Replies

  • #2
kuruman
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Homework Statement


A rod is attached to a wall in such a way it can swivel. In this case: In which direction does the force (of the wall on the rod) point to?
Here are two examples (see under attached files), but the the direction of the force is different. Why? Is maybe one of the pictures wrong?

Also: What is the recipe here? How do we determine the direction?
Different situations demand different forces having different directions. Some people think that the force is directed along the rod in such problems. That is not necessarily correct. The recipe is to find the vertical and horizontal components and get the direction from these. This is a static equilibrium problem therefore, the sum of all the forces and torques must be zero.

On edit: Should you require additional help with these problems, please post the relevant equations and your attempt at a solution as per the homework template.
 
  • #3
haruspex
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Is maybe one of the pictures wrong?
Not wrong, exactly, but image 2 is certainly misleading. The force F at the joint is shown as horizontal, but since no angles are given explicitly you should not trust that. Indeed, if you take moments about the other end of the rod then only F and mg have any torque, and both seem to be anticlockwise.

In general, if a rod is freely jointed at the ends and not subjected to forces anywhere else (so considered weightless) then the net force at each end must act along the rod. But in these examples the rods have weight.
 
  • #4
CWatters
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If the rod is fixed to the wall the force can be at any angle, as others have said.

If you have a rod learning against a frictionless wall or surface then the force can only be normal to the wall.
 

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