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Name: Peter
Status: educator
Age: 60s
Location: N/A
Country: N/A
Date: 5/24/2004


Question:
Isn't it true that, on earth, where no object really falls "freely", there is a point at which the falling object finally attains zero acceleration? That is, could you say that, in reality, a falling object attains a maximum velocity above which it will not go?


Replies:
Peter -

What you say is true. The traditional acceleration due to gravity (9.8 meters per second per second) is assuming a vacuum. However, for short falls, it works out well.

For longer falls, the force of gravity is eventually equaled by the force of air resistance. At this time the speed remains constant. As you are likely aware, in order to have a change in velocity, an unbalanced force is required.

The "terminal velocity" is determined by the shape of the object. Sky divers can speed up and slow down by changing the shape they present to the relative wind (the air going by them). Ultimately, they open their parachute and greatly increase the for of the air resistance... (luckily!) slowing them down considerably.

Larry Krengel


Yes, there is a maximum velocity an object will reach (called the terminal velocity), but gravity will always be pulling on it at an acceleration around 9.81 m/sec^2. What is causing the terminal velocity is a resistance force that is caused be the wind, or the friction of moving through air. The fall is a matter of balancing forces. The force of gravity pulling you down is counterbalanced at some point due to the force of the wind resistance trying to slow you down.

Hope this helps and thanks for using NEWTON.

Christopher Murphy, P.E.


You are absolutely correct! Gravity accelerates an object high up in the air downward at the usual rate of 32 ft/second/second. As it accelerates, however, the air resistance increases, roughly like speed at low speeds and then more like the square of the speed. Finally, the air resistance exerts an upward force exactly equal and opposite to the force of gravity. The object will then continue to fall at that speed, which is called the terminal speed.

For a person in free fall, the terminal speed is about 60 m/s or 135 miles/hour. When his parachute opens (hopefully), the terminal speed is reduced to around 12 miles/hour.

Best, Dick Plano, Professor of Physics emeritus, Rutgers University


In "theory" the answer is yes, but the answer is not simple. The main resisting force to a falling object is wind resistance(friction against the flow of air) and to a much lesser extent buoyancy (the volume of air displaced by the falling object). The former depends upon the shape of the object which determines how and if it tumbles as it falls. This obviously is a very complicated thing to predict. Even the buoyancy changes if the falling object is compressible ( i.e. a balloon). So there is a maximum speed but it is not all that simple to determine what it will be.

Vince Calder


Peter D. B.,

In open air, there is a maximum falling velocity due to air resistance. When an object moves through air, the object must push the air out of the way. In return, the air pushes back on the object. You feel air resistance when you put your hand out the window of a fast car. The faster an object moves through the air, the greater the air resistance. When falling, eventually an object speeds up enough to make air resistance as large as gravitational force. This is terminal velocity. If the object drops faster than terminal velocity, then air resistance is greater than gravity: the object slows down. If the object drops slower than terminal velocity, then gravity is the greater force: the object speeds up.

Exactly what the terminal velocity is depends on the shape, size, and density of the object. The terminal velocity of a man with an open parachute is much slower than that of a man without a parachute. It also depends on the material the object moves through. Terminal velocity of a steel ball falling through water is much slower than that of a steel ball falling through air. An object falling in a vacuum has no terminal velocity. The only limit would be due to Einstein's relativity, a completely different subject.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College



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