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Question

What is terminal velocity? Do different objects have different 'terminal velocity' speeds, or is it based on gravity/friction?
Asked by: Henry Young

Answer

As an object moves through a fluid, such as a liquid or gas there exists a drag force that acts to impede the motion. This drag force not only acts in the direction opposite to the motion but also depends on the square of the speed of the object. The fact that the drag force increases with speed has an important consequence. Consider a falling body. It is acted on by gravity and a drag force due to the air. When the body begins to fall the speed is slow and thus its drag force is also small. The acceleration of the falling body is g. As the body's speed increases so too does the drag and at some point the speed would increase tot he point where the drag force would act to cancel out the force due to gravity. Thus the body no longer accelerates and the speed remains constant.

Different objects would have different terminal speeds. The terminal velocity is not only dependent on the speed of an object but also the density of the fluid through which the object moves, the cross sectional area presented by the moving object and a drag coefficient. The cross sectional area is the area cutting across the object in a plane perpendicular to the direction of motion. The drag coefficient is a dimensionless constant and depends on the shape of the object. Thus different objects would have different terminal velocities depending on their shape and the cross sectional area they present moving through the medium as well as the acceleration due to gravity.
Answered by: David Latchman, B.Sc. Physics, University of the West Indies


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