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Free Body Diagram

The free body diagram, FBD, consists in drawing each of the bodies that appear in a problem and in wich we would like to establish its dinamic separately (a free body diagram for each), and indicate with vectors all the forces that the body receives (NOT the ones that the body makes to others... Only the ones that the body suffers, that the body receives!).

They use to be like this:

Its better not to superpose the vectors. Its not necessary for them to happen inte same point. If you superpose F2 and F3 for instance, it would be much more difficult to identify the force (the case would be different if you were working with extenese bodies, in which it does care the exact spot -inside the body- where the body acts, where the posibility of rotation is possible). What does matter is that you draw the origin of each vector inside the body which is affected by the force.

No olvides que este capítulo de la dinámica se ocupa de cuerpos puntuales

The sketch will guide you to establish the Newton's equations, , and I mean equation because if the forces aren't in the same direction, you will need two equations for each FBD, one for x and other for y, following the reference system that you decided. Then, you have to break up the forces that do not match the axis direction.,


Its better to repete the FBD after breaking it up. Here you have ir again, but with the forces that have the same direction only.

Then I can forget about forces F5 which was quite annoying and replace it for F5x and F5x.

Now, making the equations of the second law is an easier work.


           ΣFx = m ax                       F5x - F2F3 = m ax

           ΣFy = m ay                       F5y + F4F1 = m ay


In a FBD you will neverhave to draw the vectors that aren't the forces acting in the body. If you need to draw de acceleration, or more frequently the speed, do it close to it, but outside de body. Ok? Only the forces.

There is more information about the FBDs in another apunte teorico. The 1.1 problem is special for getting the idea of the FBD, I highly recommend it.
Translated by Guido Marchese. Some Rights Reserved. Not allowed to be copied without naming either the author or this source material. Last Updated may-10. Buenos Aires, Argentina.