Forces surrounding Car on a Bridge

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Homework Help Overview

The problem involves analyzing the forces acting on a stalled automobile on a bridge, specifically calculating the reaction forces at the supports due to the weight of the car. The car weighs 7500 N and is positioned one-quarter of the way across the bridge.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the concept of moments and how to calculate them about different points on the bridge. There is exploration of the relationship between the forces at the supports and the moments produced by the weight of the car.

Discussion Status

Participants are actively engaging with the concepts of moments and forces, questioning the definitions and implications of static forces on the bridge. Some guidance has been provided regarding the calculation of moments, but there is still uncertainty about the correct approach to find the reaction forces.

Contextual Notes

There is a lack of specific numerical values for the lengths involved, as the bridge length is only referred to as AB. This may affect the calculations and assumptions being made in the 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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