Mechanical energy The Work-Energy Theorem can be rewritten to incorporate potent
ID: 1536093 • Letter: M
Question
Mechanical energy The Work-Energy Theorem can be rewritten to incorporate potential energy as follows: W_ = W_c + W_ = Delta KE -Delta PE + W_ = Delta KE W_NC = Delta KE + Delta PE Here W_NC refers to work done by non-conservative forces. Examples of W_NC could include the work done by the string, the work done by a hand pushing a block, or the work done by friction. The quantity Delta KE + Delta PE is known as the change in mechanical energy. The final equation thus describes a relationship between the change in mechanical energy of a system and the work done on that system by non-conservative forces. This relationship is referred to as the Generalized Work-Energy Principle. If the work done by non-conservative forces is zero in some situation, then mechanical energy does not change, and we say that it is conserved. Discuss the equations above with your partners. Make sure you can explain each step to go from the starting equation (W_ - Delta KE) to the end result (W_SC = Delta KE + Delta PE). Listed below are the two related physical principles from this lab, stated in mathematical form. Discuss with your partners how each principle applies to the string/block situation from Activity IV. Work-energy theorem: W_ - Delta KE Generalized Work-energy principle: W_NC = Delta KE + Delta PE For the motion of the block from point 1 to point 2, was the mechanical energy of the block/Earth system conserved? Explain. Obtain your prelab. Review the questions and come to agreement with your partners on how to explain them. Annotate your original responses using colored pencil to indicate how your thinking has changed. When you are ready, check your thinking with an instructor.Explanation / Answer
a force is said to be non-conservative, if work done by the force is dependent upon the path taken.
for example, friction force is non-conservative in nature.
so when you move d meters forward and d meters back, there is zero displacement
but you have to perform work against friction force.
that is the basis of the generalized work energy principle.
i.e. when there are non-conservative forces present in the system,
work done by these forces increase the total mechanical energy of the system
and work done against these forces decrease the total mechanical energy of the system
explanation with example:
consider a block is moving on a rough surface.
its initial velocity is v m/s
its mass is m kg.
then its initial kinetic energy=0.5*m*v^2
let friction force acting on the block from the surface is F
then as it moves , it has to overcome the friction force and so it has to expend energy to perform that work
hence work done against friction force=change in kinetic energy
and finally when the change in kinetic energy becomes negative of the initial kinetic energy,
no more energy is left with the block and the block stops.
if there were no friction, there is no need to expend any energy and the block will go on moving
and the total mechanical energy will be conserved.