2016 Apr

Calculation of Air Resistance

April 1, 2016

Air Resistance

The resistance offered by air in the opposite direction to that of an object with some relative speed is called air-resistance.

Newton’s first law of motion

This states that, ‘every object retains the state of equilibrium that it’s in, unless it’s acted upon by an unbalanced external force’. This is primarily why objects in motion come to a stop when a driver is not acting to maintain their motion. Take into account a falling object on the ground from a given height. The force of gravity is an external unbalanced force which is acting on the object and this brings it to rest. Nevertheless, there’s another unbalanced external force which is acting on the body making it stop completely. This is the frictional force which is acting on the body in motion. This force friction emanates from the friction between the body in motion and the surface it’s moving on, or the friction between air molecules and the body.

This force of friction is also referred to as the drag and it’s a force that acts in the opposite direction to the object’s motion. It’s as a result of the collision of the surface of the object and air molecules. This resistance of atmosphere against forward motion of an object is also referred to as aerodynamic drag, which leads to decrease in the speed of the object. In physics, this frictional force of air that acts on a moving body is referred to as air-resistance, which is basically a type of friction.

Air Resistance Calculation Formula

As we have learnt during our Physics tuition classes, air-resistance is dependent upon the cross-sectional area as well as the speed of the object, both factors are directly proportional to the air-resistance. Nevertheless, for bodies which are smaller in size, at lower speeds, the air-resistance highly depends on the body’s speed. In this case, the of air-resistance force (Fair) is obtained as,

Fair = -cv

v = the object’s Velocity
c = Air constant.

The air constant, also referred to as the object’s drag coefficient, is a dimensionless quantity which depends on the object’s shape.

In the event of bigger objects at higher speeds, the air-resistance force (Fair) is obtained as,

Fair = -½cdAv2

c = Air constant
d = the density of air
A = the object’s cross-sectional area
v = the object’s velocity

The negative sign observed in both equations shows that air-resistance is in the opposite direction to that of the body’s motion.

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