Why don't normal forces contribute to torque in a pivoted board?

[Gabe805]

I was just curious as why, when drawing a free-body diagram of a board about a pivot with weight on either side of the pivot, we do not consider the corresponding normal forces to those weights(using torque considerations). This question was already asked in another post but was asked a little less clearly.

 

[PhanthomJay]

I was just curious as why, when drawing a free-body diagram of a board about a pivot with weight on either side of the pivot, we do not consider the corresponding normal forces to those weights(using torque considerations). This question was already asked in another post but was asked a little less clearly.

In the free body diagram of the board and weight system, you look at external forces only , the weight forces and pivot support forces. The normal forces of the weights are internal forces and excluded. If you wish , however, you can draw a free body of the board alone , then the forces external to the board become the pivot force and the normal force of the 2 objects acting on the board, and get the same result.

 

[Gabe805]

thank you.