Application of Free Fall Principle in Determining Height of Structures

RATIONALE

Introduction

Introduction to Free Fall

A free falling object is an object that is falling under the sole influence of gravity. Any object that is being acted upon only by the force of gravity is said to be in a state of free fall. There are two important motion characteristics that are true of free-falling objects:

* Free-falling objects do not encounter air resistance.

* All free-falling objects (on Earth) accelerate downwards at a rate of 9.8 m/s/s (often approximated as 10 m/s/s for back-of-the-envelope calculations)

Because free-falling objects are accelerating downwards at a rate of 9.8 m/s/s, a ticker tape trace or dot diagram of its motion would depict an acceleration. The dot diagram at the right depicts the acceleration of a free-falling object. The position of the object at regular time intervals - say, every 0.1 second - is shown. The fact that the distance that the object travels every interval of time is increasing is a sure sign that the ball is speeding up as it falls downward. Recall from an earlier lesson, that if an object travels downward and speeds up, then its acceleration is downward.

Free-fall acceleration is often witnessed in a physics classroom by means of an ever-popular strobe light demonstration. The room is darkened and a jug full of water is connected by a tube to a medicine dropper. The dropper drips water and the strobe illuminates the falling droplets at a regular rate - say once every 0.2 seconds. Instead of seeing a stream of water free-falling from the medicine dropper, several consecutive drops with increasing separation distance are seen. The pattern of drops resembles the dot diagram shown in the graphic at the right.

Next Section: Acceleration of Gravity

Jump To Lesson 6: Describing Motion with Equations

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* The twin white obelisks at the Main Campus entrance are among VSU's important landmarks that symbolize the academic programs and rural development mandate of the University.

* Both structures depict VSU's acknowledgement of providential intervention in pursuit of its programs.

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* The other obelisk with the "god of lightning and thunder" represents the technology and innovation in all its endeavors.

The Acceleration of Gravity

It was learned in the previous part of this lesson that a free-falling object is an object that is falling under the sole influence of gravity. A free-falling object has an acceleration of 9.8 m/s/s, downward (on Earth). This numerical value for the acceleration of a free-falling object is such an important value that it is given a special name. It is known as the acceleration of gravity - the acceleration for any object moving under the sole influence of gravity. A matter of fact, this quantity known as the acceleration of gravity is such an important quantity that physicists have a special symbol to denote it - the symbol g. The numerical value for the acceleration of gravity is most accurately known as 9.8 m/s/s. There are slight variations in this numerical value (to the second decimal place) that are dependent primarily upon on altitude. We will occasionally use the approximated value of 10 m/s/s in The Physics Classroom Tutorial in order to reduce the complexity of the many mathematical tasks that we will perform with this number. By so doing, we will be able to better focus on the conceptual nature of physics without too much of a sacrifice in numerical accuracy.

g = 9.8 m/s/s, downward

( ~ 10 m/s/s, downward)

Look It Up!

The value of the acceleration of gravity (g) is different in different gravitational environments. Use the Value of g widget below to look up the acceleration of gravity on other planets. Select a location from the pull-down menu; then click the Submit button.

Value of g

What is the acceleration of gravity on

(in m/s/s)?

Submit

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Investigate!

Even on the surface of the Earth, there are local variations in the value of the acceleration of gravity (g). These variations are due to latitude, altitude and the local geological structure of the region. Use the Gravitational Fields widget below to investigate how location affects the value of g.

Gravitational Fields

Enter a location and click on the Get g button.

Location:

Get g

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Recall from an earlier lesson that acceleration is the rate at which an object changes its velocity. It is the ratio of velocity change to time between any two points in an object's path. To accelerate at 9.8 m/s/s means to change the velocity by 9.8 m/s each second.

If the velocity and time for a free-falling object being dropped from a position of rest were tabulated, then one would note the following pattern.

Time (s) Velocity (m/s)

0 0

1 - 9.8

2 - 19.6

3 - 29.4

4 -