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What does it mean if there is an net force? A net force is also called an unbalanced force. Imagine you and your friend are arm wrestling. If you exert more force than your friend, you will beat your opponent. You will have applied an unbalanced force! An unbalanced force results in a change in motion, allowing you to win.
In contrast, if you each apply the same force, the forces would be balanced and there will be no change in motion.
A plane that sits on a runway has weight. The force from the ground balances the weight of the plane and there is no change in motion.
When the total force on the plane is in one direction, the force is called “unbalanced." An unbalanced force changes the motion of the plane. For instance, when thrust is greater than drag, it is the unbalanced force that causes the plane to speed up, or accelerate. In addition, as the velocity of the plane increases, the lift force increases and becomes the unbalanced force that causes the plane to fly.
If the plane exerts 3 tons of thrust, it will speed up until it experiences 3 tons of drag. During the acceleration, passengers will feel as though they are pushed back into their seats. Eventually, drag and thrust will be balanced and the velocity of the plane will be constant. In other words, when these forces are balanced, there will be no change in motion.
In fact, when a plane flies level at constant velocity, the opposite forces of flight are equal. For example, if the plane weighs 50 tons, the lift force also equals 50 tons. (Typically, lift and weight are 10 to 40 times greater than drag and thrust.)
An unbalanced force results in a change in motion. During a turbulent flight for instance, if the lift force exceeds the weight of the plane, it will lift upward. If lift decreases, weight will be the unbalanced force and the plane will fall. This is an uncomfortable experience for some passengers resulting in nausea.
As a plane begins to land, thrust is decreased. Drag becomes relatively greater and causes the plane to slow down. Lift decreases as velocity decreases. Once the plane touches the ground and decreases its velocity, lift dramatically decreases. When the plane is at rest again, the weight is the only force of flight that acts on the plane.
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