it is involved the concept of inertia ?? when a plane is flying on the sky , sometimes it will remain the constant velocity,right?and the force is balanced therefore , it does not have any force? why the plane can keep moving at the constant velocity ? due to inertia ????
There are two forces acting on the plane: air resistance trying slow it down and its engines pushing it forward against the air resistance. If it's moving at a constant velocity, these two forces are exactly balanced so the net force is zero.
if the plane need move it need force right ? so why the plane can move with a constant velocity but the force is zero?
I think you have some deep misunderstandings of the concepts of force. A force is only needed for an acceleration. This is from Newton's 1st law (and second). There is no net-force required for constant-velocity motion.
Net force is what's left over after after we've cancelled out forces that are pulling in opposite directions. You're standing on the floor, gravity is pulling you down against the floor and the floor is pushing back against you and the upwards force balances the downwards force (if it didn't, you'd fall through the floor, the same way that you can't walk on water because water under your feet won't push you up with the same force that gravity is pulling you down). So we get zero net force when two equal forces act in different directions and cancel each other out. Zero force does not mean zero speed; it means zero change in speed. That's what Newton's ##F=ma## is telling us.
There are several forces acting on an airplane flying through the air. One is gravity of the earth which is tying to pull the plane downwards. Another is the lift from the wings. If these 2 forces are balanced, then the plane will stay in the air at an altitude. Increase the lift then the plane will climb upwards. Decrease the lift and the plane will descend. Since the weight of the plane, from gravity, is mainly constant, the pilot has to use his controls to make the plane climb from takeoff or descend for a landing. What the pilot is doing is changing the vertical net force on the plane. As an equation this would be, for the vertical direction(s), F[itex]_{net}[/itex] = F[itex]_{lift}[/itex] - F[itex]_{gravity}[/itex] You can see from the equation that when F[itex]_{net}[/itex] = 0 the plane will stay at a constant altitude. Or, ascend or descend if the forces from the lift or gravity are not zero. You can also see that even when the vertival net force is equal to zero, this does not mean there are no forces acting on the plane. We can think about it in the same way for the velocity of the plane in the horizontal direction. We have 2 forces - the drag from the air, and the thrust from the engines. Decrease the thrust and the plane will slow down. Increase the thrust and the plane will speed up. Keep the engine thrust the same as the air drag and the plane will fly at a constant velocity. Then at a constant velocity, the horizontal net force = 0, but we still have forces acting on the plane, except these horizontal forces, the thrust and the air drag are equal to one another.