The discussion revolves around the predictability of the nature of Normal Reaction (NR) in mechanical setups, specifically addressing whether it is possible to determine its magnitude and direction without solving the dynamical equations of motion. Participants explore intuitive understandings versus mathematical requirements in various mechanical contexts.
Participants express disagreement on whether the nature of Normal Reaction can be predicted without solving dynamics. While some assert that intuitive predictions are possible, others maintain that mathematical analysis is essential for accurate determination.
Limitations include the dependence on specific mechanical setups and the complexities introduced by varying material properties and surface interactions, which may affect the predictability of Normal Reaction.
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.Dale said:No. Sometimes you need to actually do the math to get the magnitude and direction.
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.kuruman said:the direction of that normal force exerted on the system is perpendicular to the surface and away from it
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.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.
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.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 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:Normal Reaction
If you know the exact deformations and material properties, then you can predict the contact forces without knowing 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?
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.A.T. said:If you know the exact deformations and material properties, then you can predict the contact forces without knowing the dynamics.
I realize that this is probably tangential, but the so-called normal force is actually the perpendicular component of the contact force. Thus as @Dale points out it is by definition always perpendicular to the surface. (The parallel component is the so-called friction force.)kuruman said: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.