sometimes we find the net external work done by the system n equate it to change in potential energy and sometimes we equate it to the change in kinetic energy . and sometimes with total kinetic energy and potential energy . can u tell please that when should we use which equation to get the result .

If the work is done in motion of the system such as ball moving then equate it to KE and if in rest i.e. by the virtue of its position such as a car in rest then equate the net work done to PE. But if the body does work due to its position and motion i.e the body is falling from a height then net work done is equated to the total KE and PE. A freely falling body has both KE due to its motion and Pe due to its position from the Earth. :)))))

Approved