Forces surrounding Car on a Bridge

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SUMMARY

The discussion centers on calculating the additional reaction forces at supports A and B of a bridge due to a stalled automobile weighing 7500 N, positioned one-quarter of the way across. Participants clarify the concept of moments, emphasizing that the moment produced by the reaction force at B (RB) is calculated as the force multiplied by its perpendicular distance from point A. The correct approach involves taking moments about point A to find the relationship between the forces and distances involved, leading to the conclusion that the moment produced by the weight of the car must be balanced by the moments at the supports.

PREREQUISITES
  • Understanding of static equilibrium in structures
  • Knowledge of moment calculations in physics
  • Familiarity with forces and reaction forces in bridge mechanics
  • Basic principles of rotational motion
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  • Study the principles of static equilibrium in engineering mechanics
  • Learn about calculating moments and their applications in structural analysis
  • Explore the concept of reaction forces in bridge design
  • Review examples of force distribution on beams and bridges
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Students in engineering, physics enthusiasts, and professionals involved in structural analysis and bridge design will benefit from this discussion.

Oktane
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Homework Statement


An W = 7500 N automobile is stalled one-quarter of the way across a bridge (see Fig. P77). Compute the additional reaction forces at supports A and B due to the presence of the car. Take the length of the bridge to be AB.

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Homework Equations





The Attempt at a Solution


I was told that if you pick one side of the bridge as your rotation point, then either A or B is already a length of 0. But I'm not sure how to proceed from here, any help would be greatly appreciated!
 
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Call the reaction at A, RA and at B, RB.

So if you take moments about A, then the moment of RA about A is 0 right?

So what is the moment produced by RB about the point A and what is the moment produced by W about A?
 
I didn't know what a moment was so I read the explanation but doesn't that mean that the force around point B would just be a normal force of 7500 because the object is static? But that isn't the right answer.
 
Oktane said:
I didn't know what a moment was so I read the explanation but doesn't that mean that the force around point B would just be a normal force of 7500 because the object is static? But that isn't the right answer.

a moment is the turning effect produced by a force which acts perpendicular to a body.

So the moment of RB about A, would be the force multiplied by its perpendicular distance from A.
 
Oh ok! That makes sense, however the bridge length is only given to be AB. So that would mean, if A was the rotation point, that A=0 and therefore B=7500AB right?
 
Oktane said:
Oh ok! That makes sense, however the bridge length is only given to be AB. So that would mean, if A was the rotation point, that A=0 and therefore B=7500AB right?
No the force is RB and the distance is AB , so the moment produce is (AB)*RB Right?

Can you find the moment produced by the 7500N force?
 
rock.freak667 said:
No the force is RB and the distance is AB , so the moment produce is (AB)*RB Right?

Can you find the moment produced by the 7500N force?

Would that be the rotational acceleration around point A?
 
Dr Meow said:
Would that be the rotational acceleration around point A?

Taking moments about A.
 

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