How to find the forces acting on the pivot point ?

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Discussion Overview

The discussion revolves around determining the forces acting on a pivot point located at the middle of a ramp, specifically focusing on the reactions at the pivot due to applied forces and the impact of the ramp's angle. The context includes homework-related problem-solving in mechanics.

Discussion Character

  • Homework-related
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant presents a calculation of moments about the pivot point and derives reaction forces Ra and Rc, questioning the significance of the angle of the ramp.
  • Another participant asserts that the reactions are both 250 lbs and states that the angle does not affect the forces as long as the system remains symmetric, assuming vertical forces.
  • A later reply emphasizes that there are only force reactions at the pivot pin, leading to a vertical shear force, and mentions that moments can be taken about either point A or point C.
  • Further inquiries are made about how to find the deflection of the beam, with a suggestion to use material properties and reference formulas for beam deflection.

Areas of Agreement / Disagreement

Participants generally agree on the nature of the reactions at the pivot but have not reached consensus on the implications of the ramp's angle or the method for calculating deflection.

Contextual Notes

Some assumptions regarding the symmetry of the forces and the verticality of the applied loads are not explicitly stated. The discussion also involves unresolved steps in calculating deflection, depending on material properties and beam geometry.

JJ3
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Homework Statement


pivot point located at the middle of ramp( Point C).
F1=250 lbs @ 12 in from point A
F2=250 lbs @ 84 in from point A
L=192 in
angle from the ground is 25 degrees

Homework Equations


sum of moments equal to 0
sum of forces equal to to 0

The Attempt at a Solution


I did sum of moments about point c equal to 0 , solve for Ra
0=(F1*84)+ (F2*12) - (Ra*96)
Ra= 250 lbs in
Same thing for Rc
0=-(F1*12)-(F2*84)+(Rc*96)
Rc=-250 lbs in
My question is does the angle make a big difference?
 

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The reactions are both 250 lbs (not lbs in). The angle makes no difference as long as the symmetry stays the same because all forces are assumed vertical.
 
Gordon Mutten said:
The reactions are both 250 lbs (not lbs in). The angle makes no difference as long as the symmetry stays the same because all forces are assumed vertical.
Thank you for the reply.
One more question How would the moment acting on point C affect the pivot pin?
 
There are no moment reactions, only force reactions. Therefore the pivot pin is subjected to a vertical shear force only. You can take moments either about point A or Point C because the sum of the moments at these positions is zero.
 
Gordon Mutten said:
There are no moment reactions, only force reactions. Therefore the pivot pin is subjected to a vertical shear force only. You can take moments either about point A or Point C because the sum of the moments at these positions is zero.
one last question how would i go about finding the deflection on the beam
 
JJ3 said:
one last question how would i go about finding the deflection on the beam
For this you need the E and the I of the beam section. That is the Youngs Modulus of the beam material and the second moment of area that controls bending in the vertical plane. Then I would look up the case of a simply supported beam with a point force in Roark's Formulas for Stress and Strain (Table 3 case 1e in my 5th Edtion). This will give you a formula for the deflection of a beam with one point force. So you need to do the calculation for one point load and then the other and algebraically add the results. I recommend an online tool for doing this as follows: http://www.amesweb.info/StructuralBeamDeflection/SimpleBeamTwoConcentratedLoadsAtAnyPoint.aspx
Treat the beam as if it were level and shorten the lengths by multiplying them by cos(angle).
 

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