Air resistance is the slowing effect that air has on a body moving through the atmosphere, caused by the collision of a solid object with gas molecules. It affects free-falling objects, aircraft, and vehicles moving at high speed. Galileo discovered that lighter objects are slowed down by air resistance, while heavier objects have sufficient weight to counteract it. The greater the surface area and speed of an object, the greater the air resistance. Parachutists use air resistance to control their descent, while aircraft and vehicles are designed to overcome it.
Air resistance, also called wind resistance, is the slowing effect that air creates on a body as it moves through the atmosphere. For example, free-falling objects or people will find their descent slowed by the air. It is also a factor in the movement of aircraft or any vehicle moving at high speed. This resistance has a variety of other effects, some of which are easily observed.
Mankind has always been able to observe the effects of air resistance, but the physical factors involved weren’t understood until the 17th century. Galileo, trying to understand the principle of gravity, used experiments to test Aristotle’s thesis that heavier objects fall faster than lighter ones. He was able to demonstrate that this was not true; the gravitational force affects every object equally. He realized that lighter objects were slowed down by air resistance and that heavier objects had sufficient weight to counteract this.
Air resistance is caused by the collision of a solid object with gas molecules in the atmosphere. The greater the number of air molecules, the greater the resistance. In practice, this means that an object with a larger surface area encounters greater resistance. A faster object also has greater air resistance because it contacts more air molecules in a given amount of time. When the drag of a free-falling object equals the force of gravity on the object, it no longer accelerates. This is called terminal velocity and varies depending on factors such as weight, surface area and velocity.
The effect can be observed by watching the paratroopers in action. Before activating his parachute, the skydiver falls at extreme speed, seemingly held aloft by the air. If he retracts his limbs and points his body down, his speed will increase as the resistance decreases. By positioning his body parallel to the ground and spreading his arms and legs, he can slow his descent. Once the parachute opens, the air resistance will increase, slowing it even further. The terminal velocity of the open parachute is low enough to hit the ground at a sustainable speed.
Aircraft are designed to overcome air resistance, which is called drag in the field of aerodynamics. The aerodynamic design of most jet aircraft and rockets allows them to pass through the atmosphere with the least possible resistance. Automobiles and trains also use aerodynamic designs to a lesser extent for the same purpose. Unless they’re designed for high-speed travel, air resistance isn’t as significant an obstacle for land vehicles as it is for aircraft. Semi-trucks sometimes have curved roofs to reduce drag in the space between the truck and trailer, which can have a negative effect on gas mileage.
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