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Object at rest or object that is moving with constant speed has same free body diagram (FBD).

How do I know looking only at FBD is object moving or at rest?

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You don't. The FBD can give you the acceleration, but not much more. Suppose the acceleration is zero, then depending on your inertial frame, the object could be either moving at a constant velocity or be at rest. To conclude something about the motion based on FBD, you'd need more info, such as an initial condition.

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  • $\begingroup$ You mean I need initial condition? $\endgroup$
    – Jurgen M
    Nov 28 '21 at 8:36
  • $\begingroup$ Exactly. Because what you're really doing is solving a differential equation, and to get a particular solution (i.e. to know the velocity as a function of time), you need at least one IC. @JurgenM $\endgroup$
    – user256872
    Nov 28 '21 at 9:04
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Free body diagrams are mainly used to represent forces, acceleration, velocity etc related to the body (or system). They help us to understand the given information in a better manner.
After making the diagram, now you have to understand the situation and perform required calculations as asked by the question.

You can interpret FBDs as a snapshot of the body (system) taken at some particular time. At that time forces, acceleration, velocities related to the body is represented in the diagram and then you perform calculations.

Hope this helps

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  • $\begingroup$ There are no velocities in a FBD. $\endgroup$
    – nasu
    Nov 29 '21 at 11:29
  • $\begingroup$ @nasu Marking physical quantities is totally up to you. There is no certain rule which states which quantities you have to mark and which not. $\endgroup$ Nov 30 '21 at 8:15

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