Free Body Diagram Title

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This text is meant to accompany class discussions. It is not everything there is to know about the forces. It is meant as a  prep for class. More detailed notes and examples are given in the class notes, presentations, and demonstrations (click here.)

Fill out this summary sheet as after reading each section, (click here). (Your teacher may give you a Google Apps link instead.


Using Free Body Diagrams to Develop a Math Model

The ultimate purpose of a free body diagram is to develop a math model to answer a question. This math model will look like a set a equations. In order to be successful at this, the you must know when each force in the previous section applies to a problem and in which direction is points. Below is summary.

Force Name
Math Symbol
Formula When it is present Direction
weight
w
w=mg
On any big object, like a planet. Always down -to the planet's center
normal force
...
when two surfaces touch perpendicular to the surface
Friction
When the problem indicates parallel to the surface & opposite the direction of motion
Tension
T
...
When ropes, chains or an object being stretched is involved parallel to the rope. You can't push with a rope.
Net force
Fnet
...
When a body is changing speed or direction In the direction of the acceleration
This list will grow as the course progresses. You must know the information in this table to be successful at free body diagrams.

 

How to draw free body diagrams

This video gives basic instructions and shows 3 examples.

This video can be found on YouTube at: http://youtu.be/Ve7lWyGIIqc

 

Example
  • Question
  • Answer

Two trucks have a rope connecting their under carriages.The begin to have a "Tug of War." The trucks drift to the right at a constant velocity. Draw a free body digram for each truck and the rope.

 

To develop the math model a set of equations will be created. The set of equations varies from problem to problem.

To create a equation the force will be added in one or more directions. This is called summing and the math symbol that is used to indicate this is the capital Greek letter sigma, "Σ." If the forces are going to be summed in the x-direction, then the summation looks like the one shown below.

To indicate that the forces are added up in the y-direction the symbol below is used.

The next step is to show what these summed forces will equal. They will either equal zero or a net force. If the body is moving at a constant velocity then the summation will equal zero. (Recall that a body at rest is moving at a constant velocity of zero.) If the body is accelerating, then the summation will equal Fnet or as it is sometimes written, manet.

Constant Velocity
Acceleration
...If it accelerates in the "x" direction.
...If it accelerates in the "y" direction.

These summations do not need to be the same. One may be equal to zero and the other may be equal to manet.



 

 


by Tony Wayne ...(If you are a teacher, please feel free to use these resources in your teaching.)