A sailboat is floating across a lake. Four forces act on the boat: its weight, the buoyant force (the contact force with the water that pushes the boat up), the forward force of the wind, and the backward drag of the water. At a given instant, the force of the wind on the boat and the drag with the water are both equal to 100 N, and since the lake is perfectly smooth, the boat moves across the water without bobbing up and down.