Can someone please explain this problem to me

[doubled]

Homework Statement

https://docs.google.com/a/rams.colo...&sig=AHIEtbQGcbZbLABawi6f0FTNFXdQt_G2CA&pli=1


That's the link to the problem. It's problem 4/67. I don't understand why in the FBD in my solution manual they took away the 30lb force pointing up and have the force on the circle thing pointing in the +x and -y direction. Can someone explain this to me please? Thanks.

 

[PhanthomJay]

Homework Statement

https://docs.google.com/a/rams.colo...&sig=AHIEtbQGcbZbLABawi6f0FTNFXdQt_G2CA&pli=1


That's the link to the problem. It's problem 4/67. I don't understand why in the FBD in my solution manual they took away the 30lb force pointing up and have the force on the circle thing pointing in the +x and -y direction. Can someone explain this to me please? Thanks.

since the force on the pin is internal, you have to 'take apart' the pliers in a 'free body diagram' of one of the 2 pieces, in order to solve for the internal forces. When you do this, the other 30 lb force is not in the picture. The force of the circle thing on that part of the pliers is perpendicular (normal) to the contact point, and can be broken into its horizontal and vertical components.

 

[doubled]

Oh thanks a lot. So if we look at the top part wouldn't the 30lb force pointing down be applying a force on the circle that should be pointed in the -x and +y direction not the opposite? Like if I had a pair of pliers and I only press down on the upper that would make the mouth push up.

 

[PhanthomJay]

Oh thanks a lot. So if we look at the top part wouldn't the 30lb force pointing down be applying a force on the circle that should be pointed in the -x and +y direction not the opposite?

Well yes, that's Newton's third law, the force of the pliers on the circle act to squeeze it . But when you draw the FBD of the pliers, the force of the circle thingy acting on the pliers points in the +x and -y direction (down and to the right), which you can show by summing moments about the pin.

if I had a pair of pliers and I only press down on the upper that would make the mouth push up.

Well, you can't just press down on the upper part, you have to sqeeze the handles together with an equal but opposite force on each in order to make the thing work. The lower jaw moves up, and the upper jaw moves down, as if it were biting the 'circle' (a nut or bolt or rod, etc).