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A net force will always create displacement which approaches infinity if left undisturbed. So how to fix a value of displacement in work formula? Isn't a mention time interval necessary? Or do we measure displacement only while force is acting on the body i.e the moment we stop applying force we also stop measuring displacement.

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Usually problems are framed with "before" and "after" states. A book was here, a force applied, and later the book was there. So there might be an implicit time interval, but that interval is not used in calculating the work. Also, frequently, the object starts at rest and ends at rest, also providing a time interval, which again is not used in calculating work. And the question can be phrased where the position of the object changes and continues to change. In that case the question must specify an initial position and ending position between which work is calculated. If you know how the position varies with time, $\vec{r}(t)$ then those positions can be found from a starting time and ending time.

But in any event, it is always the displacement that counts. Time would be involved only as a means to identify the starting position and the ending position.

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  • $\begingroup$ Summing up, you're almost always going to calculate work done between points A and B. If you want to know how much work is being done every second, that's called "power". $\endgroup$ – André Chalella Jun 5 '14 at 21:14

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