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Terminal Velocity: Free Falling

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Author: Janice VanCleave

The term "free fall" is commonly used to indicate the condition of an object falling toward Earth. Examples include a parachutist in the part of his or her jump before the parachute opens, or a falling ball. In physics, an object is said to be in free fall if the only force acting on the object is gravity. An object doesn't have to be falling "straight down" to be freefalling; the only requirement is that gravity is the only force acting on it. So when a ball is thrown up, even when it is rising, it is in free fall. Once it leaves your hand, if you neglect air resistance (the retarding force of air on objects moving through it), gravity is the only force acting on the ball.

In this project you will determine the effect of the shape and weight of a falling object on acceleration. You will calculate the terminal, or final, velocity of an object in free fall and compare it to the actual terminal (final) velocity of an object falling through Earth's atmosphere. You will also determine the effect of drag (the force of resistance on an object moving through a fluid) and weight on terminal velocity.

Getting Started

Purpose: To determine the effect of shape on the acceleration of falling objects.

Materials

  • 2 basket-type coffee filters
  • ruler

Procedure

  1. With your hands, slightly spread the sides of one of the coffee filters so that the diameter across its top is 1 to 2 inches (2.5 to 5 cm) greater than that of the other filter. Leave the sides of the other filter as vertical as possible.
  2. With their open sides up, hold the filters, one in each hand, as high as possible and at the same height, then drop the filters at the same time. Determine which filter falls faster.
  3. Repeat step 2 three or more times.

Results

The filter that is less spread out falls faster.

Terminal Velocity: Maximum Velocity in a Fluid

Why?

Velocity is the speed and direction of a moving object. Acceleration is a change in velocity per unit of time. Free fall is the motion of an object when the only force acting on it is gravity. In Earth's gravitational field (the region of space in which a force of gravity acts on objects), free fall takes place near Earth's surface at a constant acceleration of 32 ft/sec2 (9.8 m/sec2), and is known as acceleration of free fall. However, since Earth is surrounded by an atmosphere (blanket of gases surrounding a celestial body–natural objects in the sky, such as planets), falling objects collide with air molecules that exert an upward retarding force caused by the friction between the air and the surface of the object. (Air is the name for the mixture of gases in Earth's atmosphere.) This frictional force is called drag (the retarding force acting on an object moving through fluid, such as air or water). The greater the surface area of the falling object, the more air molecules the falling object strikes per second. An increase in the number of air molecules striking the surface of the falling object increases the upward force or drag on the object. With an increase in drag, the acceleration of the falling object decreases, as indicated by an increase in the time it takes the object to fall.

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