Basic concepts
Terminology
- Force
- It is whatever has the capacity to change velocity. Forces do not necessarily change velocity because they can be counteracted by other forces acting in the opposite direction. Forces are how objects interact with one another.
- Normal force
- The word ‘normal’ means perpendicular to a surface. A normal force is a reactionary force
induced when an object attempts to move through a solid surface
directed perpendicular to the surface and opposite to the direction of attempted motion. - Force of friction
- This is also a reactionary force
induced when two solid surfaces either move of attempt to move against each other. When they are actually moving, it is called ‘kinetic friction’, and when they just attempt to move, it is called ‘static friction&rsuqo;.
directed parallel to the surface and opposite to the direction of motion or attempted motion. - Restoring force
- A reactionary force that is
induced when a solid object is deformed (i.e., its shape is changed)
directed in such a way that aims to restore the original shape of the solid object.
A restoring force is an internal force; i.e., it builds inside the object being deformed. - Tension force
- A restoring force that is induced when a solid object is stretched (elongated). It is always directed toward the middle of the object. If the object is light enough, we can consider the magnitude of the tension force within the same object to be the same at any point of the object.
- Compression force
- A restoring force that is induced when a solid object is compressed (squeezed). It is always directed away from the middle of the object. If the object is light enough, we can consider the magnitude of the compression force within the same object to be the same at any point of the object.
- Applied force
- A generic term that means ‘do not worry where the force is coming from’. It does not imply anything in particular.
Fundamental forces
These are the basic forces by which we can explain all other forces we see in the universe. Whether they happen or not depends on the nature of the objects involved. There are four fundamental forces
- Gravitational forces. These are how objects with mass interact with one another.
- Electromagnetic forces. These are how objects with electric charge interact with one another.
- Weak nuclear forces
- Strong nuclear forces
The nuclear forces are how subatomic particles interact with one another. They are not studied at this level.
System diagram versus free-body diagram
| System diagram | Free-body diagram (FBD) |
|---|---|
| Can be drawn in any problem, not only those involving forces | Drawn specifically in problems involving forces |
| Aimed at showing the ‘big picture’; i.e., the interaction between the various objects of the system and their possible interactions with their surroundings, as well as their orientation and relative positions. | Aimed at analyzing the forces acting on any one object in the system. |
| We typically draw just enough information to provide maximum clarity. Any piece of information can be drawn on a system diagram. | We draw all forces that act on a given object, and those forces only. We do not include any other quantities, except probably acceleration because it is strongly related to the net force. Even then, we visually distinguish acceleration from forces. |
| There is typically one system diagram per problem. Occasionally, we may need to draw more than one if we need to view the system from different perspectives. | We draw one FBD for every object of interest in the system. |