While the cart is moving along an aisle, it comes in contact with a smear of margarine that had recently been dropped on the floor. Suddenly the friction force is reduced from -40.0 newtons to -20.0 newtons. What is the net force on the cart if the “pushing force” remains at 40.0 newtons? Does the grocery cart move at constant velocity over the spilled margarine? Identify the normal force on the shopping cart after 75 newtons of groceries are added to the cart.