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 .