When Does the Kinetic Work Energy Theorem Not Apply?

[uestions]

When does the Kinetic Work Energy Theorem not apply to a situation? Or better, is there a general form of the equation where work can equal the change in any energy? What is work besides a force and a distance?

 

[TysonM8]

When does the Kinetic Work Energy Theorem not apply to a situation?

The Work-Energy Theorem only applies to rigid bodies. That is, if the work is not used to deform the object.

is there a general form of the equation where work can equal the change in any energy?

There's a thread here that discusses this in detail;
http://physics.stackexchange.com/questions/58134/how-to-understand-the-work-energy-theorem

What is work besides a force and a distance?

Work by definition, is what a force does on an object by displacing it. However, there are other ways of representing work if that's what you're asking.

 

[Doc Al]

The Work-Energy Theorem only applies to rigid bodies. That is, if the work is not used to deform the object.

Here's something that I wrote in another thread that may clarify how the "work"-energy theorem, when thought of as an application of Newton's 2nd law, may be applied to deformable bodies.

The so-called 'work'-Energy theorem is really an application of Newton's 2nd law, not a statement about work in general. Only in the special case of a point mass (or rigid body) is that "work" term really a work (in the conservation of energy sense).

If you take a net force acting on an object (like friction) and multiply it by the displacement of the object's center of mass, you get a quantity that looks like a work term but is better called pseudowork (or "center of mass" work)--what it determines is not the real work done on the object, but the change in the KE of the center of mass of the object. This is usually called the "Work-Energy" theorem:
$$F_{net}\Delta x_{cm}=\Delta (\frac{1}{2}m v_{cm}^2)$$
Despite the name, this is really a consequence of Newton's 2nd law, not a statement of energy conservation.

 

[uestions]

How can work be done to an object that has a change in potential energy, but no change in velocity?

 

[Doc Al]

How can work be done to an object that has a change in potential energy, but no change in velocity?

If the velocity doesn't change, the work-kinetic energy theorem just says that the net work must be zero. You do work when you lift an object at constant speed, but gravity is also doing negative work.