Torques on a door with hinges

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  • #26
haruspex
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how do you manage to get two equations for vertical
One from the force balance, sum=mg, and the one they gave you, that the two are equal. You already combined these to obtain the answer: 2 * Fy=mg.
So you already have all the answers. You know the vertical and horizontal components at each hinge. All that remains is deciding the form in which to write answer.
If you have not been taught a vector notation then that leaves two possibilities. Either just list components (FAx=... etc.) or calculate the magnitude and direction of each.
 
  • #27
iamnotsmart
One from the force balance, sum=mg, and the one they gave you, that the two are equal. You already combined these to obtain the answer: 2 * Fy=mg.
So you already have all the answers. You know the vertical and horizontal components at each hinge. All that remains is deciding the form in which to write answer.
If you have not been taught a vector notation then that leaves two possibilities. Either just list components (FAx=... etc.) or calculate the magnitude and direction of each.

The problem is that I am not allowed to just write it as F_y=mg/2 because m is not listed in the task, therefore Fg is still unknown... And I have no idea how to find Fg as using the other equations will make Fg cancel out anyways (I might have choked though, but I tried several times and still can't find out how to find Fg expressed by the terms given in the task.)
 
  • #28
haruspex
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m is not listed in the task
So how were you able to answer the earlier parts? You need the weight of the door to find the torques.
 
  • #29
iamnotsmart
So how were you able to answer the earlier parts? You need the weight of the door to find the torques.

I found an expression of the torque from Fg around point A and B, however I just defined it as Fg, thinking I'll manage to find an expression for Fg later in the task. For example I found out that torque around point A is: (L/2)*F_B_x - (d/2)*Fg = 0 where L/2 is the distance from A to B and F_B_x is the x-component of B that is perpendicular to the distance from A to B. (d/2)*Fg is the torque from gravity around point A as far as I am concerned, I just take the radius vector component that is perpendicular to the gravity force (I was told by the teacher that F*r*sin(theta) is the same as taking either the force component that is perpendicular to r times r, or the component of r that is perpendicular to F times F.) As you see, I only express it as Fg, thinking I would be able to find an expression of Fg later to solve for the vertical component of the hinge forces.
 
  • #30
haruspex
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I found an expression of the torque from Fg around point A and B, however I just defined it as Fg, thinking I'll manage to find an expression for Fg later in the task. For example I found out that torque around point A is: (L/2)*F_B_x - (d/2)*Fg = 0 where L/2 is the distance from A to B and F_B_x is the x-component of B that is perpendicular to the distance from A to B. (d/2)*Fg is the torque from gravity around point A as far as I am concerned, I just take the radius vector component that is perpendicular to the gravity force (I was told by the teacher that F*r*sin(theta) is the same as taking either the force component that is perpendicular to r times r, or the component of r that is perpendicular to F times F.) As you see, I only express it as Fg, thinking I would be able to find an expression of Fg later to solve for the vertical component of the hinge forces.
Ok, but without a given mass or weight for the door there is no hope of finding any torques or forces. Are you quite sure there is nothing given? Any diagram?
 
  • #31
iamnotsmart
Ok, but without a given mass or weight for the door there is no hope of finding any torques or forces. Are you quite sure there is nothing given? Any diagram?

So the professor said he forgot to say that we can express our Fg with the mass of door m, therefore Fg isn't unknown anymore and I can express everything with the terms m,g,d and l now, thanks for your time :)
 

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