In inclined Net acceleration for a body down an incline is given by F=mgsintheta – f.(1)But for upward motion,net acceleration is given by F=mgsintheta + f(2)Why there is no negative sign in equation 2 as friction is oppsitive and also mgsintheta and f acts oppostive to it.

It is because when the body is going down the inclined plane the forces mgsintheta and f are opposite to each other and hence the force required to just move it is F=mgsintheta-f.

While when the body is going up the inclined plane the two forces mgsintheta and f are acting in the same direction, we need the force that is just able to move the body and must be equal to the forces which are opposing the required motion. The force applied upward is equal to the forces acting on the body in opposite directions which are mgsintheta and f.

Therefore, F=mgsintheta+f.

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