The friction between the solid objects and the air
When a solid object moves in the air, a friction force arises between the object and the air, This type of the friction is called the air resistance and it acts in the opposite direction of the body movement, So, it resists its movement.
The air resistance
The object is falling through the air experience a type of friction called the air resistance, As the surface area increases, the air resistance increases, and eventually the air resistance equals the gravity.
The greatest velocity an object reaches is called the terminal velocity, and in the vacuum, All the objects fall at the same rate, The speed of the body and the surface area of the body are the factors affecting the air resistance, Where the air resistance increases when the velocity and the surface area of the body increases and vice versa.
The air resistance increases, by increasing the speed of the car or a bicycle, But to overcome this resistance, more energy is exerted and more fuel is used, So, Reduce the speed of the car to a certain limit to reduce the air resistance and the consumption of the fuel.
When the air resistance affecting a moving car equals the force that results from the increase in the speed of the car, the car moves with a constant velocity.
The bodies of the birds have streamline shapes to decrease the air resistance, and they stretch their wings on landing to increase the air resistance by increasing their surface area and this causes a decrease in their speed on landing.
The parachutist should open the parachute to land safely to increase the air resistance by increasing its surface area and accordingly the falling speed decreases, The trains, the rockets and the aircrafts are designed in streamline shapes to decrease the air resistance.