Calculating Forces Acting on a 50m Bridge with Two Supports: A Torque Question

[kc0ldeah]

Homework Statement

http://imgur.com/BqIoe

There is a 50 meter bridge with two supports. The bridge has a mass of 80000kg. A truck of 30000 kg is 15m from one of the supports. What are the forces acting on the ends of the bridge?

Answers in back are F_a(support closer to truck): 598000 N and F_b: 480000 N

Homework Equations

torque = Force_perpendicular * distance

center of mass for uniform object is center of geometry

The Attempt at a Solution

Sum of Forces in y direction on bridge: = -80000g - 30000g + F_n A + F_n B = 0

Torque @ A = -15(30000) - 25(80000) + 50*F_n B = 0
F_n B = 490000

Torque @ A = 35(30000) + 25(80000) - 50*F_n A = 0
F_n A = 305000

What am I doing wrong? Any help is appreciated.

 

[kc0ldeah]

Bump? Anyone?