# Homework Help: Forces surrounding Car on a Bridge

1. Jan 11, 2010

### Oktane

1. The problem statement, all variables and given/known data
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.

2. Relevant equations

3. 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!

2. Jan 11, 2010

### rock.freak667

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?

3. Jan 11, 2010

### Oktane

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.

4. Jan 11, 2010

### rock.freak667

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.

5. Jan 11, 2010

### Oktane

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?

6. Jan 12, 2010

### rock.freak667

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?

7. Jan 12, 2010

### Dr Meow

Would that be the rotational acceleration around point A?

8. Jan 13, 2010