Here is a small aeroplane. It is flying. At any given time four forces are acting on it.
Most of us know about weight or gravitation force. It is a force which pulls everything down. This force is pulling the aeroplane down. This is not good from point of view of flying. I mean, without gravitation all of us could fly!
When we run fast, we feel air pushing us back. We can experience it much more easily while riding a bike. This force is called air drag. Our aeroplane also goes very fast. Air drag pushes it backward. This force tries to slow the aeroplane down. This is bad. We want to go fast. However, it exists. We have to deal with that.
Let us come to good forces. Engine force pulls the aeroplane forward. This force is generated by the engine (the fan!) in front. The engine pushes the air back. The air pushes the engine (and hence aeroplane) in forward direction. All aeroplanes have engines. In a small aeroplane, the engine is in front. In a bigger aeroplane, there is more than one engine. They are located on wings. Jet engines look a bit different. The ‘fan’ is not visible from side.
Lift force acts on wings in upward direction. We will discuss its origin as well. Pilots fly aeroplanes by adjusting these forces. To go faster, engine force should be greater than drag force. To slow down, engine force has to be reduced. To go up, lift force should exceed weight. To go down slowly, lift force should be slightly less than weight. (The lift force should not be removed. The aeroplane will crash!)
Bernoulli Family
The Bernoulli family was based in Basel in Switzerland. They are considered one of the most intelligent families in human history. More than ten members of the family have made extraordinary contributions to mathematics, physics, astronomy, geography and architecture.
Daniel Bernoulli had a mathematician father and a mathematician uncle. Both were famous. They had made major contribution to Calculus and Probability in Mathematics. However, Daniel’s father did not want him to become a Mathematician (“There is no money in Mathematics!”). He wanted Daniel to study Business or Medicine!
Daniel agreed to study medicine provided his father taught him mathematics privately. He became a doctor, but then he became interested in physics! He excelled in physics, mathematics as well as medicine. He is best known for Bernoulli Principle. This principle explains implication of energy conservation on motion of fluids.
Pressure
Pressure is a force which presses from all sides. Pack too many clothes in a suitcase. The clothes will be pressed by other clothes from all sides. We can say that clothes are feeling pressure. We can experience pressure in an overcrowded bus or train.
Air also applies pressure. At any given time, our atmosphere is pressing us from all sides. This pressure is called atmospheric pressure. It is quite big.
Why don’t we feel it? Two reasons. First of all, the pressure is applied from all sides. The net force is zero. The second reason is that we are used to it. The working of our lungs (breathing) depends on atmospheric pressure. If pressure disappears, it will be quite noticeable. By noticeable I mean suffocation followed by death. We will notice it if that happens.
Bernoulli Principle
Daniel Bernoulli studied flow of fluids. One of the results he obtained was that, pressure decreases when liquid or gas travels at high speed. Fast moving air has lower pressure!
There are many applications of Bernoulli Theorem. Some of the applications include swing of ball in cricket, bending of football in air and curve-ball in baseball.
Let us look at an example which is easy to understand. In India during marriages and many other events, pandals are set up. Pandal (or a marquee) is the large tent made up of fabric which is used to host parties and other events. Let us assume we have set up a pandal. Wind starts blowing. What happens? The fabric of the pandal blows up in outward direction. Try to observe it next time. Pandals are quite common in India. It is easy to observe them swelling outside whenever the wind blows rather wildly.
What happens here? Air outside the pandal moves fast because of wind. Air inside the pandal cannot move that fast. As a result, outside pressure becomes less than inside pressure. Remember, faster air will have lower pressure. High pressure air from inside causes the fabric of pandal to move outwards.
Aeroplanes
Wings of aeroplane are called aerofoil. It has got an interesting shape when we look at it from side. When an aeroplane travels at high speed, air moves past the wings. The wing slices the air in two streams. These streams merge together behind the aeroplane. Shape of the aerofoil ensures that the upper stream has to travel a longer distance compared to the lower stream in the same time.
Upper stream has to travel faster. As per Bernoulli Principle, faster stream should have lower pressure. Pressure above the wing decreases. Higher pressure from lower side causes an upward force on the aeroplane. This upward force is lift. This force is enough for the aeroplane to fly!
This is a highly simplified explanation for lift force. Its advantage is that it is easy to understand. Things become a little more complicated when we try to calculate these forces.
