Forces surrounding Car on a Bridge

AI Thread Summary
The discussion revolves around calculating the additional reaction forces at supports A and B of a bridge due to a stalled 7500 N automobile positioned one-quarter of the way across. Participants clarify the concept of moments, emphasizing that the moment at point A is zero since it is the pivot point. The moment produced by the weight of the car and the reaction force at B is calculated using the force and its perpendicular distance from point A. There is confusion regarding the static nature of the car and how it affects the forces at the supports. Ultimately, the focus is on understanding how to apply the principles of moments to find the reaction forces accurately.
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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