Terminal Velocity (1 of 3)

Terminal Velocity: The velocity at which the driving forces are cancelled out by the resistive forces. Terminal velocity depends a great deal upon the shape of the object that is facing the direction it is moving. Once an object has reached terminal velocity, the object is not accelerating (a=0), therefore it is not speeding up or slowing down. It is a constant velocity unless the driving forces or the resistive forces change. Typically, Terminal Velocity is only a possibility when you are dealing with fluid friction as opposed to contact friction like static or kinetic friction.
Driving Forces: Forces that try to cause motion: In the case of falling it would be gravity (weight). In the case of a car it would be the force from the engine (through the friction on tires). Sometimes an object can have multiple driving forces. (ie. an airplane in a dive, has the engine pushing it down at the same time gravity is pulling it down)
Resistive Forces: The forces that try to resist motion. The force of fluid friction (from a liquid or gas) plays the main role in creating terminal velocity.
Fluid Friction: Fluid friction differs from contact friction because the amount of fluid friction depends on how fast the object is moving through the fluid. The greater the speed, the greater the friction. This can be felt if you are in a pool of water. Trying to walk from one side of the pool to the other is much easier than trying to run. That's because the faster you move, the harder the fluid pushes against you. When you examine contact friction you find that speed has no effect on the amount of kinetic friction.

In the movie above, it is important that you notice the following things: (use the arrow buttons on the right side of the movie to go just one frame at a time)

Initially the only force acting on the books is the force of gravity (their weight). Only as the books start falling does the force of air friction (drag) begin acting on the books.
The faster the books go, the greater the force of air friction.
Even though the two books are identical in mass and shape, the one with more surface area facing the direction of motion reaches its terminal velocity first.
The terminal velocity of each book can be seen on the graph when the velocity vs. time graph shows no increase (remains flat). It can also be seen by watching the digital velocity display. When the velocity no longer increases, the book has reached terminal velocity.

Š1999 Science Joy Wagon