When all three types of energy change, the law of conservation of energy for an

Question

When all three types of energy change, the law of conservation of energy for an object of mass m can be written as

12mv2i+mghi+12kx2i=12mv2f+mghf+12kx2f.

The gravitational force and the elastic force are two examples of conservative forces. What if nonconservative forces, such as friction, also act within the system? In that case, the total mechanical energy would change. The law of conservation of energy is then written as

12mv2i+mghi+12kx2i+Wnc=12mv2f+mghf+12kx2f,

where Wnc represents the work done by the nonconservative forces acting on the object between the initial and the final moments. The work Wnc is usually negative; that is, the nonconservative forces tend to decrease, or dissipate, the mechanical energy of the system.

If the zero level is a distance 2h/3 above the floor, what is the potential energy U of the block on the floor?

Explanation / Answer

As here zero level is assumed at a height 2h/3 so on the ground height is -2h/3

and potential energy on the ground will be mgh

U = mg(-2h/3)

U = - 2mgh/3

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