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Air Resistance

In real life there are a lot factors that affect motion. When first learn physics it is much simpler to calculate the motion of an object if forces such as drag and friction are taken out of the equation because it simply makes understanding motion much easier. In this section we will be exploring the concept of air resistance in motion.

A Mathematical Model

Air resistance is a force that essentially opposes motion and dissipates energy. Much like other opposing forces, air resistance is dependent on both the speed and the size of the surface area of the object. Many things go into what affects the force of air resistance, and it can be defined by the following equation:

p = density of the air C_D = drag coefficient (typically between .3 and 1.0) A = cross-sectional area v = speed of object WHY AIR RESISTANCE DEPENDS ON C_D

WHY AIR RESISTANCE DEPENDS ON CROSS-SECTIONAL AREA It can be seen from practice that the bigger the cross-sectional area of the object the larger the effect that air resistance has on the object. This due to the fact that air resistance is the result of the collision of an objects surface with the air molecules. This means that the bigger the surface-area the more collisions with air molecules the object will experience and the faster it'll reach terminal velocity. For example a person going sky diving will fall much slower with an open parachute (more surface area, air resistance has a bigger impact) than with a closed parachute (less surface area, air resistance has a smaller impact).

WHY AIR RESISTANCE DEPENDS ON SPEED Air resistance is affected by speed because it increases as velocity increases. This can be seen because there is always gravitational force acting on the object downward, but as the speed increases the air resistance increase making the net downward force much much smaller until it becomes 0. This means that at some point the object reaches terminal speed because there is no longer a net force acting upon it.

Examples

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