5.12:
Drawing Free-body Diagrams: Rules
Suppose a man has to push an object up an inclined surface. He should apply a force to balance other forces so that it does not topple over. All the forces on the object can be graphically visualized using a free-body diagram.
For this, replace the object by a simple alternative like a square or a circle, or a point in the case when its shape and size are not important.
Replace each force with a line arrow pointing in the same direction as the force and having a size that reflects its magnitude. Include only those forces that affect the system and not those exerted by the system on the surrounding.
In our present example, the refrigerator is the object of interest. So, in the free-body diagram, the refrigerator is drawn as a square.
The forces acting on it are the gravitational force due to its mass, the normal force, the frictional forces due to the inclined surface, and the force applied by the man. The force exerted by the refrigerator is not considered.
5.12:
Drawing Free-body Diagrams: Rules
The first step in describing and analyzing most phenomena in physics involves the careful drawing of a free-body diagram. Free-body diagrams are useful in analyzing forces acting on an object or system, and are employed extensively in the study and application of Newton's laws of motion. The steps to draw a free-body diagram are listed below:
- • Draw the object under consideration. If treating the object as a particle, represent the object as a point. Place this point at the origin of a xy-coordinate system.
- • Include all forces that act on the object, representing these forces as vectors. However, do not include the net force on the object, or the forces that the object exerts on its environment.
- • Resolve all force vectors into x– and y-components.
- • Draw a separate free-body diagram for each object in the problem.
Once a free-body diagram is drawn, Newton's laws of motion can be applied to solve the problem. Newton's first law is applied if the body is in equilibrium, whereas Newton's second law is applied if the body is accelerating.
Note: If there is acceleration, it is not directly included in the free-body diagram; however, it may be helpful to indicate acceleration outside the free-body diagram. It may be labeled in a different color to indicate that it is separate from the free-body diagram.
This text is adapted from Openstax, University Physics Volume 1, Section 5.7 Drawing Free-Body Diagrams.