Final answer:
The forces acting on an airplane moving at constant speed include thrust, lift, drag, and weight. At constant speed, these forces balance out so there is no net force, allowing the plane to fly straight and level.
Step-by-step explanation:
When an airplane is moving at a constant speed, there are several forces acting on it, which can be understood through the principles of Newton's Laws of Motion and analysis of free-body diagrams. In this specific case, the forces include both thrust and lift as well as weight (due to gravity) and drag (air resistance). Thrust is the forward force provided by the airplane's engines that allows it to move through the air, while lift is the upward force that counteracts the airplane's weight and allows it to stay aloft. Both of these forces work against drag and weight, respectively, which are the opposing forces that act on the airplane. The forces of thrust and drag act horizontally, while lift and weight act vertically. At constant speed, thrust and drag are balanced, as are lift and weight, meaning the net force in both the horizontal and vertical directions is zero.