Displacement using work energy principle How do you use the work energy principle to find the displacement of an object. With of course the mass, speed and friction forces given?
 A: In general you cannot use the work-energy principle to find the displacement of an object.
The principle you are referring to is given by
$$W_{net}=\Delta K$$
Since you are asking about displacement, I will assume you know the change in the kinetic energy. Then you are really interested in the net work:
$$W_{net}=\int_{x_1}^ {x_2}\mathbf F_{net}\cdot d\mathbf x$$
In general, the net force can involve non-conservative forces, so that the total work done depends on the path taken and not just on the displacement. Even if you had all conservative forces, the forces themselves could have dependencies on position such that a given displacement doesn't give a unique amount of work done.
You can find displacement this way in certain situations, but since your question doesn't give any specific situations, I will stop here for now. I can edit accordingly if you give more specifics and if this answer is not sufficient for those specifications.
A: You cannot use this principle to find the displacement.  Consider for instance a particle moving with constant linear velocity $v$ and thus subject to no force.  Clearly no work is done on the particle as there is no force acting on it, but also clearly the displacement after $\Delta t$ will be just $\Delta x=v\Delta t$.
