PLMGSS-ACE Li ShiYu's Science Blog

Monday, February 23, 2009

[[Terminal velocity]]

What is Terminal Velocity?

Terminal velocity is the term for the state an object reaches when the force of drag acting on it is equal to the force of gravity acting on it. When an object reaches its terminal velocity, it no longer accelerates, remaining at whatever velocity it was already traveling or else slowing down.

As an object accelerates, the amount of drag exerted on it increases. This means that more force is necessary to sustain the same level of acceleration. If that external force is increasing, as in a car or plane, then the object can be accelerated well past its terminal velocity. If, however, the only force being exerted on it is the force of gravity, then eventually the drag will become as great as the static force of gravity, and the object will cease to accelerate.

An object may also decelerate towards terminal velocity, if it was initially moving faster than terminal velocity. This may be because it entered from somewhere with less drag, such as the thinner upper atmosphere, or because it was initially launched with some external force other than gravity at a greater velocity. In this way, terminal velocity can be viewed as a sort of equilibrium point that objects in freefall naturally gravitate towards.

Strictly speaking, an object never actually reaches its terminal velocity, it simply reaches a state which approximates it. Instead, in a manner similar to Zeno’s paradox of motion, the object comes closer and closer to its terminal velocity, reducing its acceleration to miniscule amounts, until the acceleration is no longer even measurable or functional, and terminal velocity is said to have been achieved.

Examples

Based on wind resistance, for example, the terminal velocity of a skydiver in a free-fall position with a semi-closed parachute is about 195 km/h (120 mph or 55m/s).[1] This velocity is the asymptotic limiting value of the acceleration process, since the effective forces on the body more and more closely balance each other as the terminal velocity is approached. In this example, a speed of 50% of terminal velocity is reached after only about 3 seconds, while it takes 8 seconds to reach 90%, 15 seconds to reach 99% and so on. Higher speeds can be attained if the skydiver pulls in his limbs (see also freeflying). In this case, the terminal velocity increases to about 320 km/h (200 mph or 90 m/s),[1] which is also the terminal velocity of the peregrine falcon diving down on its prey,[2]. And the same terminal velocity is reached for a typical 150 grain bullet travelling in the downward vertical direction — when it is returning to earth having been fired upwards, or perhaps just dropped from a tower — according to a 1920 U.S. Army Ordnance study.[3]
Competition speed skydivers fly in the head down position reaching even higher speeds. The current world record is 614 mph (988 km/h) by Joseph Kittinger, set at high altitude where the lesser density of the atmosphere decreased drag.[1]
An object falling toward the surface of the Earth will fall 9.81 meters per second faster every second (an acceleration of 9.81 m/s²). The reason an object reaches a terminal velocity is that the drag force resisting motion is directly proportional to the square of its speed. At low speeds, the drag is much less than the gravitational force and so the object accelerates. As it accelerates, the drag increases, until it equals the weight. Drag also depends on the projected area. This is why things with a large projected area, such as parachutes, have a lower terminal velocity than small objects such as cannon balls.
Mathematically, terminal velocity, without considering the buoyancy effects, is given by

where
Vt = terminal velocity,
m = mass of the falling object,
g = gravitational acceleration,
Cd = drag coefficient,
ρ = density of the fluid through which the object is falling, and
A = projected area of the object.
On Earth, the terminal velocity of an object changes due to the properties of the fluid, the mass of the object and its projected cross-sectional surface area.
For objects falling through the atmosphere, air density increases with decreasing altitude, ca. 1% per 80 m (see barometric formula). Therefore, for every 160 m of falling, the terminal velocity decreases 1%. After reaching the local terminal velocity, while continuing the fall, speed decreases to change with the local terminal velocity.

[[ Branched ]]*|2/23/2009 01:58:00 PM|

[[ Branched Over Me ]]

Name:shi yu
Birthday: 7th of march
School:Payar Leber MGS
Contact:

[[ My Adores ]]

Fruits:strawberry,mango,honey dew
Drinks:peach tea
Pastimes:reading manga
People:secret!

[[ Music's Playing ]]

Artist:
Song:

[[ My History ]]

[[ The Conversations ]] >

Board

here----->

[[ My Friends ]]

melissa
lisa
radiant
jasley
stella
lishan
sarah
janelle
stacy's latest blog
stacy's previous blog
abigail
seow hwee
dorcas
sophia
joyi
rachel
[[ Credits ]]