It isn't easy to lift a 15 lb (67 Newtons) watermelon up and into the grocery cart, yet you can easily push the cart containing the melon through the aisles. Energy and direction of force explain the difference. To lift the watermelon you have to apply a 67 Newton force to balance the force of gravity on it, and you have to apply that while the melon moves upwards 1/2 meter. Force x distance is the amount of energy transferred. But when you push the loaded cart, you are not exerting 67 N -- hardly any force at all, if the cart has good wheels -- and the energy you use is small, even when pushing it for a long distance. Meanwhile, the downwards force of gravity on the watermelon is being balanced by an upwards force by the floor. If the floor is level, the cart is not moving upwards or downwards, and neither the floor nor gravity is transferring energy.