Air Pressure Effects the Speed of Falling Objects

Research

An object that is falling through the atmosphere is subjected to two external forces. The first force is the gravitational force, expressed as the weight of the object. The weight equation which is weight (W) = mass (M) x gravitational acceleration (A) which is 9.8 meters per square second on the surface of the earth. The gravitational acceleration decreases with the square of the distance from the center of the earth. If the object were falling in a vacuum, this would be the only force acting on the object. But in the atmosphere, the motion of a falling object is opposed by the air resistance or drag. The drag equation tells us that drag is equal to a coefficient times one half the air density (R) times the velocity (V) squared times a reference area on which the drag coefficient is based.

The motion of a falling object can be described by Newton's second law of motion, Force = mass x acceleration. Do a little algebra and solve for the acceleration of the object in terms of the net external force and the mass of the object (acceleration = Force / mass). The net external force is equal to the difference between the weight and the drag forces (Force = Weight - Drag). The acceleration of the object then becomes acceleration = (Weight - Drag) / mass. The drag force depends on the square of the velocity. So as the body accelerates, its velocity (and the drag) will increase. It will reach a point where the drag is exactly equal to the weight. When drag is equal to weight, there is no net external force on the object, and the acceleration will become equal to zero. The object will then fall at a constant velocity as described by Newton's first law of motion. The constant velocity is called the terminal velocity.

What is aerodynamics? The word comes from two Greek words aerios concerning the air, and dynamis, meaning powerful. Aerodynamics is the study of forces and the resulting motion of objects through the air. Humans have been interested in aerodynamics and flying for thousands of years, although flying in a heavier-than-air machine has been possible only in the last hundred years. Aerodynamics affects the motion of a large airliner, a model rocket, a beach ball thrown near the shore, or a kite flying high overhead. The curve ball thrown by big league baseball pitchers gets its curve from aerodynamics.

If both friction and air resistance were eliminated from acting on the swinging pendulum, would gravity act on the pendulum to slow it down and eventually stop? If all friction and air resistance was eliminated (plus, the losses due to deformation of the string and the like) the pendulum, under ONLY the effect of gravity, would keep swinging indefinitely. This is because gravitation is a 'conservative' force; it does not drain any energy from the object moving under it, it just converts the energy from one form to another. When the pendulum reaches either end at its highest point, all the energy is potential energy, and the kinetic energy is zero. At the bottom of the swing, the kinetic energy is maximum, while potential energy is minimized. No energy is transferred out of

the system, so it must keep moving. In the presence of friction, however, energy is removed from the system in the form of heat. (The air heats up a little and the contact point of the string heats up a little).

What is air resistance? Basically, it is friction between an object and the air. What causes air resistance? All matter is made from atoms and/or molecules. The air is no exception. When something moves through the air, it bumps into the atoms and molecules. Take for example a car: Air particles hit the front of the car as it travels through the air. Even though atoms and molecules are very tiny and light, each collision causes a force on the moving object (the car). The force from each individual collision is, therefore, very tiny. There are however millions of these collisions each second, so millions of tiny forces add up to make a large overall force. All of these little forces, all in the same direction, equal one big force. This big force is called air resistance. What happens to the size of the air resistance force as the speed increases? If the car is going faster, then it will hit the atoms and molecules of the air harder. This means that the tiny forces will be bigger. This in turn means that the air resistance force gets bigger as something moves faster. This can be felt happening when walking and running. When walking, unless it is windy, one does not normally notice the air resistance because it is very low (collisions are not very hard). When running, one can readily feel the air resistance on the face.

It has already been established that for a ball falling through water, Aristotle's description is closer to the truth than Galileo's "naturally accelerated motion". Galileo was well aware of that, and his response was that the true natural motion could only take place in a vacuum, otherwise, the medium the body was falling through would impede the acceleration to some degree. Galileo's insight was that for a reasonably heavy body falling a few meters through air, the air resistance did not make a big difference. He did state explicitly, though, that even for heavy materials, air resistance was important at the speeds attained by firearms.

In practice, as Galileo understood, the air resistance of a falling object increases with speed. But he did not think in terms of forces, and the force of gravity, the weight, pulling the ball down. So he thought