Term for air resistance?

Question

We measured the time it takes for an object to hit the ground from a certain height. We have initial velocity, we have initial position, and acceleration. Using Intregration we know how long it should take the object to hit the ground. How do we use the difference between our observed time and the ideal time to have a term for air resistance?

Answer 1

Let's assume you're in the low subsonic regime so the equations are still 'relatively' simple.



The basic formula for the time t taken for an object mass m to fall from height h, accelerated by gravity g is t = √(2 s/g). The force downwards on the body is m g.



Air resistance implies an upward force counteracting the force due to gravity. When these two forces cancel, the body is no longer accelerating and has reached what we call its 'terminal velocity'. The low velocity drag equation says that the force on a falling object depends on the square of its velocity v, its cross sectional area A (the area it presents 'head-on' as it falls), the density ρ of the fluid through which it is moving, and a dimensionless parameter called the 'drag coefficient'. Putting it all together, we get an equation for the drag force Fd



Fd = (ρ v² Cd A)/2



The net force on the falling object is m g - Fd so it's easy to find the terminal velocity vt when mg = Fd



vt = √((2 m g)/(ρ Cd A))



Actually solving the equation of motion taking air resistance into account is probably beyond elementary calculus; suffice to say that the solution involves the hyperbolic tangent function tanh().

Answer 2

There is no definite way to calculate the air resistance an object will experience. It depends too much on the shape and size of the object. Air resistance, or drag force, tends to be proportional to the square of the speed of the object (relative to the air). It is also proportional to the mass density of the air. If the shape os "aerodynamic", like a plane wing or a javelin, air resistance is less. If the front surface of the object is large and flat, air resistance is more. Air resistance of a amooth ball is less than that of a rough cube, provided both are about the same size and traveling at the same speed through the same air. In addition to air density, humidity can have an effect on certain materials.

http://www.newton.dep.anl.gov/askasci/phy00/phy00297.htm



Air Friction

Air friction, or air drag, is an example of fluid friction. Unlike the standard model of surface friction, such friction forces are velocity dependent. The velocity dependence may be very complicated, and only special cases can be treated analytically. At very low speeds for small particles, air resistance is approximately proportional to velocity and can be expressed in the form F = -BV

http://hyperphysics.phy-astr.gsu.edu/hbase/airfri.html

Answer 3

In aeronautics it's called "drag".