Calculating Counterweight for Platform Stability

[sailsinthesun]

Homework Statement

The platform assembly has weight 250lb and a center of gravity at G1. If it is intended to support a maximum load of 400lb at G2, determine the smallest counterweight W that should be placed at B in order to prevent the platform from tipping over.

http://img252.imageshack.us/img252/7074/problemgq2.th.jpg http://g.imageshack.us/thpix.php

Homework Equations

Sum Fx=0
Sum Fy=0

The Attempt at a Solution

I don't know where to begin.

 

[Stovebolt]

Start with a diagram. If one was provided, please try to post it here.

It sounds like this is a problem where you want to prevent the platform from rotating under a load, so you will need to make the sum of the moments = 0. Think of it as a lever - how much force is needed to maintain balance under the given load.

 

[sailsinthesun]

Sorry I guess the picture I posted didn't show up. I tried another hosting site, is it showing up now?

 

[Stovebolt]

Yes, the diagram is showing up now. Thank you, it is much easier to follow.

In order to keep this assembly from tipping over, the center of gravity must be somewhere between points C and D. If we overload it at G2, it will tip over in a clockwise direction with point D as the center of rotation, so we should use point D to determine the sum of the moments.

Because all the loads are applied vertically, the heights at which they are applied are not important - just the horizontal distance from point D.

Let me know if this gets you started, or if you need me to clarify.

 

[sailsinthesun]

Yes, the diagram is showing up now. Thank you, it is much easier to follow.

In order to keep this assembly from tipping over, the center of gravity must be somewhere between points C and D. If we overload it at G2, it will tip over in a clockwise direction with point D as the center of rotation, so we should use point D to determine the sum of the moments.

Because all the loads are applied vertically, the heights at which they are applied are not important - just the horizontal distance from point D.

Let me know if this gets you started, or if you need me to clarify.

So using equilibrium equations we get Sum of moments about d=0 and (400lb*2ft)-(7ft*W)-(250lb*1ft)=0 then solve for W correct?

 

[Stovebolt]

So using equilibrium equations we get Sum of moments about d=0 and (400lb*2ft)-(7ft*W)-(250lb*1ft)=0 then solve for W correct?

Sounds right to me.

 

[sailsinthesun]

Thanks a lot. :)