I To predict the nature of Normal Reaction in any mechanics set-up

AI Thread Summary
In mechanical systems, the normal reaction force is defined as the force exerted perpendicular to a surface, typically directed away from it. While the direction of this force is generally understood to be perpendicular, predicting its magnitude and direction without solving the dynamics can be challenging, especially in complex setups like a trampoline with multiple interacting bodies. Some argue that knowing material properties and deformations can allow for predictions of contact forces, but this is not universally applicable. In many cases, the dynamics must be analyzed to accurately determine the normal force. Ultimately, intuition alone is insufficient for predicting normal reactions in all mechanical scenarios.
rahaverhma
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Is it true that in any mechanical set-up, it is possible to predict the nature of Normal Reaction ( magnitude, direction, etc. ) without solving through the dynamical equations of motion and constraints for the set-up as Normal Reaction is completely unknown? I mean is it true that we can explain NR intuitively beforehand?
 
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No. Sometimes you need to actually do the math to get the magnitude and direction.
 
Dale said:
No. Sometimes you need to actually do the math to get the magnitude and direction.
I think it is safe to say that, if a system is in contact with a surface providing an external normal force, the direction of that normal force exerted on the system is perpendicular to the surface and away from it. In short, you cannot pull but you can push with a surface.
 
kuruman said:
the direction of that normal force exerted on the system is perpendicular to the surface and away from it
Yes, but there are mechanical setups where the direction of the surface is not known until after you solve the dynamical system. For example, consider two kids jumping on a trampoline.
 
Dale said:
Yes, but there are mechanical setups where the direction of the surface is not known until after you solve the dynamical system. For example, consider two kids jumping on a trampoline.
I am not sure I understand what you are talking about. I can understand that a 2D surface has two normals which makes the direction undefined. However, the normal force is a contact force. When one asks about the direction of a contact force, it is tacitly assumed that there exists a system which is in contact with the surface. If there is no such system the normal force is zero. ##N=0## is the condition that we teach as meaning that contact is lost and when that happens the kids' feet are off the trampoline.

Thus, I am not convinced that the direction of the normal force is not always the perpendicular to the surface that points towards the system. After all, the electrons in the kids' feet are repelled by the electrons in the trampoline surface.
 
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kuruman said:
I am not convinced that the direction of the normal force is not always the perpendicular to the surface that points towards the system. After all, the electrons in the kids' feet are repelled by the electrons in the trampoline surface.
The normal force is always perpendicular to the surface. That is a matter of definition and has nothing to do with electrons. The normal force is defined as the perpendicular force.

But when you have two kids on a trampoline you don’t know which direction is perpendicular to the trampoline surface until you solve the dynamics of the whole system.
 
rahaverhma said:
Normal Reaction
The normal reaction force is the locally perpendicular component of the contact force; the paralllel component is conventionally called the frictional force. These are names (definitions) which are helpful for clear description but not directly for specification of a particular system. No matter what they are called, one usually needs to solve the dynamics.
 
rahaverhma said:
Is it true that in any mechanical set-up, it is possible to predict the nature of Normal Reaction ( magnitude, direction, etc. ) without solving through the dynamical equations of motion and constraints for the set-up as Normal Reaction is completely unknown? I mean is it true that we can explain NR intuitively beforehand?
If you know the exact deformations and material properties, then you can predict the contact forces without knowing the dynamics.
 
A.T. said:
If you know the exact deformations and material properties, then you can predict the contact forces without knowing the dynamics.
But there are many systems that you don’t know the deformations without knowing the dynamics. And further there are surfaces for which knowing the deformations is insufficient. Things like dense viscoelastic materials.
 
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