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History : Chute Types : Newton's Laws : Drag Force : Terminal Speed : Real/Model Chutes               
MOVIE 1:   6 minutes 17 seconds

Terminal Speed

Most anyone not schooled in classical physics would claim that heavy things always fall faster than lighter things. The NASA movie shows one of the Apollo astronauts demonstrating from the surface of the moon the fall of a feather and a heavy object dropped at the same time. They hit the surface of the moon at the same moment. How can this happen?

Newton asked the opposite question: isn’t this what you should expect, unless something interferes? Newton realized that to make a clean description of how a body moves, you have to imagine what it will do with no friction or drag, and then put friction back into the picture as a force like any other.

Air drag affects the picture in a special way because it’s a force that increases with speed. This means that for a given object falling through air, there will be a speed at which its air drag (an upward force) is as strong as its weight (which is pulling it downward). Once it reaches this speed, so that these two forces cancel, there will be no net force to accelerate it further, so (by Newton’s first law) it continues at this speed, which is called its terminal speed.

Things whose weight is small for their size, like feathers, will have low terminal speeds -- they will not fall fast in air because even at a low speed their air drag will become strong enough to cancel their weight. This effect creates the familiar experience that heavy things fall faster.

The moon has no atmosphere and so air drag does not exist there. On the moon, Newton's second Law of Motion (click link to see earlier page for more information), rules during the entire freefall of an object on the moon -- the body accelerates until it reaches the surface of the moon. With the force due to the moon's gravity working unimpeded, the feather and heavier object speed up at the same rate. (One common question people have is why a larger mass, which experiences a greater force of gravity, does not speed up faster than a lighter object. The short answer is that the greater mass' increased inertia offsets the increased force of gravity, leaving the accelerations of a lighter and heavier body equal.)

Look at the picture to the right. Suppose the two parachutes are the same size but one has twice as heavy a load. When the lighter and heavier loads fall, they start off accelerating or increasing their speed downward because gravity's force downward is greater than the drag force upward. At some speed, let's say 1 m/sec, the drag force for this size of chute equals to the force of gravity on the lighter load and its parachute. The net force on this parachute system then is zero, so this chute continues to travel at the speed when it first had a net force of zero -- 1 meter per second. At this speed the more heavily-loaded parachute experiences the same drag, but that drag force does not yet equal its greater force of gravity. It still has a net force downward, and so it continues to increase in speed. When it reaches a new speed, 1.4 m/sec, the drag force equals the force of gravity and now the heavier chute is at its own terminal speed.

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