Help with moment/force question `

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Discussion Overview

The discussion revolves around solving a moment and force question related to torque calculations in a mechanical context. Participants explore the application of torque equations, force vectors, and equilibrium conditions in their reasoning.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • Some participants suggest summing torques about the contact point using the equation T = r x F, with specific vector representations for r and F.
  • One participant calculates r as 250i + 75j + 300k and expresses uncertainty about proceeding from the moment equation (75*z)-(300*y)=280Nm.
  • Another participant emphasizes the need to convert units from mm to m and notes that F is perpendicular to the spanner, leading to the expression r x F = 280i.
  • A participant proposes that if the force points in the -j direction, the equation simplifies to (0.075*0)-(0.30*-Fj) = 280Nm, leading to a calculated force F of 933.33 N.
  • One participant suggests drawing a free body diagram and applying equilibrium conditions for further analysis.
  • Another participant mentions using the formula M=F*D, indicating a distance calculation involving the square root of the sum of squares of dimensions, but later questions the relevance of one dimension in determining the force.
  • A final response suggests that the correct force should be about 905 N, indicating a potential correction to earlier calculations.

Areas of Agreement / Disagreement

Participants express differing views on the relevance of certain dimensions and the application of formulas, indicating that multiple competing approaches exist without a clear consensus on the final answer.

Contextual Notes

There are unresolved assumptions regarding the dimensions and their impact on the force calculation, as well as the conditions under which the force acts perpendicular to the spanner.

shaka091
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Hey guys I was hoping you would be able to solve this question for me with a worked solution :)

nd1o2r.jpg


And also this question, thanks a lot :)

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Last edited:
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You just need to sum the torques about the point where it makes contact.

T = r x F

F points in the opposite direction of the y-axis so F = -Fj.

To get r just write down how you get from the point to the force.

Example: If you move along the x-axis 30 mm positively and you reach the force r= 30i mm
 
rock.freak667 said:
You just need to sum the torques about the point where it makes contact.

T = r x F

F points in the opposite direction of the y-axis so F = -Fj.

To get r just write down how you get from the point to the force.

Example: If you move along the x-axis 30 mm positively and you reach the force r= 30i mm

Ok so I r=250i + 75j + 300k

And F is unkown. Because the moment is around the x axis, I got something like this (75*z)-(300*y)=280Nm

But wouldn't know where to go from there.
 
shaka091 said:
Ok so I r=250i + 75j + 300k

And F is unkown. Because the moment is around the x axis, I got something like this (75*z)-(300*y)=280Nm

But wouldn't know where to go from there.

The unit of r is mm, convert that to m.

F is perpendicular to the spanner. If the spanner is vertical (parallel to z) then the force is parallel to y. It is pointing in the -j direction so that F= -Fj

and r x F = 280i
 
Ok so, r=0.25i + 0.075j + 0.30k

because the force points in the -j direction I got, (0.075*0)-(0.30*-Fj) = 280Nm

which then got F=933.33 ?
 
Yes that should be correct.

Draw the free body diagram for the second problem and use the same set of equilibrium conditions:

∑Fx = ∑Fy = ∑Fz = ∑M = 0
 
Thanks for your help, lol turns out I should have just used this formula M=F*D

with distance being (300^2+250^2)^0.5 * F = 280
 
shaka091 said:
Thanks for your help, lol turns out I should have just used this formula M=F*D

with distance being (300^2+250^2)^0.5 * F = 280

Not quite.
The 250 mm dimension is not relevant. This dim could be made larger or smaller without changing the force (F) required.
The problem states "Assume that Force (F) acts perpendicular to the spanner". This greatly simplifies the problem.

Final answer should be about 905 N.
 

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