Theory type Question of Static Equilibrium

[AVQueen87]

Hi everyone!

I just have a quick question about the theory behind a concept in statics.

Problem:
I was given a question that states that a sign is attached to a pin joint and also to a spring. The other end of the spring is attached to a spring joint as well. They hang there in static equilibrium and the spring is stretched from its initial position. I am to find the spring constant, k.

The picture looks like the one http://docs.google.com/viewer?a=v&q...JST4T&sig=AHIEtbR68GGRJvMjfoLJqjT_T1_ms1pCSg" in problem 41 (page 220) but the joints are instead pin joints.

Attempt:
I know that there will be normal forces between the pin joint and the sign. I also know how to find the tension force in the spring that is attached to the sign and

Question: This is more of a fundamental type thing I guess. My professor says that there are normal forces at both pin connections. Why is the connection between spring and the pin joint creating normal forces? I originally thought that normal forces would only occur between a rigid body and connection (like the pin connector and the sign). Any clarification on this theory would be greatly appreciated!

Thank you

 

[tiny-tim]

Welcome to PF!

Hi AVQueen87! Welcome to PF!

My professor says that there are normal forces at both pin connections. Why is the connection between spring and the pin joint creating normal forces? I originally thought that normal forces would only occur between a rigid body and connection (like the pin connector and the sign).

At a contact point between two things, there will always be a https://www.physicsforums.com/library.php?do=view_item&itemid=73" , and if the contact is loose (eg no glue or screws) that will be a normal force if there is no friction.

The spring is presumably connected to the pin joint by a hook on the end of the spring …

why would there not be a normal force between the pin joint and the hook?

 

[AVQueen87]

Mmm... I see now. So, for the majority of question in textbooks they just draw a spring/string/whatever-type-of-connector directly to the wall which makes it fixed, not like a pin connection. That's probably where I drew the (wrong) assumption that normal forces occur on rigid bodies - because I've never seen a scenario like this one!

Because this spring was connected to a pin joint, it will behave just like anybody connected to a pin joint... OK!

Thanks for the clarification, tiny-tim :) I can let my mind rest now, lol!