Moments

Hi,
I wanted to understand the meaning of freely hinged and freely suspended in the context of moments. My intuition tells me that if someone tells us that a door is freely hinged, it means that the door can easily rotate without any friction between the inner and outer parts of the hinge. Although, if someone tells us that a lamina is freely suspended at a point, this means it is allowed to rotate freely without any obstacle. I think there is no friction acting at the point of suspension on lamina. Am I right about that?

Hi,
I wanted to understand the meaning of freely hinged and freely suspended in the context of moments. My intuition tells me that if someone tells us that a door is freely hinged, it means that the door can easily rotate without any friction between the inner and outer parts of the hinge. Although, if someone tells us that a lamina is freely suspended at a point, this means it is allowed to rotate freely without any obstacle. I think there is no friction acting at the point of suspension on lamina. Am I right about that?

Yes you are basically correct.

This simplifies your modelling since you do not need to take into account additional friction that oppose rotation and would require a greater moment to overcome.

All sounds good. A hinge restricts linear motion but allows rotational about a particular axis.

Can there be friction at point of suspension of a lamina?

Generally suspended would mean freely syspended (usually in equilibrium), so no friction.
Is there a particular question?

So you want to say, that there maybe certain friction acting at the point of suspension of a lamina?

Of course, though if it is suspended then more often than not there is barely any friction.

A better example is an old rusty hinge on a door -- it will definitely oppose motion with with frictional internal forces (hence more difficult to open) but if you oil it up then thin layers of lubrication will allow slippage between the internal components and hence reduce friction.

Let's honest, you will not be asked to model anything moderately realistic at A-Level so you dont need to worry about it.

Yeah you are right about that. I just wanted to make my understanding better. I think the friction will only act if we are moving the lamina along the point of suspension, just like when a vehicle moves along a road. And in most cases on college level there is no friction.

That scenario would be complex. If the lamina was moving it would probably be on a rough table, rather than a point.

k I see. That would definitely make the scenario more complex.

Point of syspension would typically be used if you had a bar supported by two points of suspension. Depending on where the com was, it could be in equilibrium or rotate about either point. When the reaction at one point of suspension becomes negatuve, you'd have linear motion away from it.

A hinge would not permit this.