Question

if in an elevator an atltwood system is kept and the elevator is going up with an accel a , what is effective gravityand why is g-a

Answer 1

For an elevator going up with acceleration $a$, the effective gravity is $g+a$. The formula $g-a$ represents the effective gravity when the elevator is accelerating downwards with acceleration $a$.

Answer 2

Effective gravity ($g_{eff}$) is the apparent acceleration due to gravity felt by an object in an accelerating frame of reference. For upward acceleration $a$, $g_{eff} = g+a$. For downward acceleration $a$, $g_{eff} = g-a$. Thus, $g-a$ is effective gravity for downward acceleration, not upward.