Free fall motion

One important elementary example of motion is that of free fall, which is the motion of an object only under the influence of gravity. It is a remarkable fact, first discovered over 300 years ago by Galileo and others, that objects in free fall motion descend at the same rate, independent of their mass, as long as the effects of air resistance can be neglected. This means that under free fall all objects have the same constant acceleration, which in the metric system is 9.8 m/s² at sea level, directed towards the center of the earth. One fairly dramatic illustration of this fact can be done by taking a fairly large text book (say the one for this course), and a piece of paper that is just smaller than the pages in the book. If you hold the book in your left hand and the sheet of paper in your right, then drop them simultaneously, it is quite evident that the two do not fall towards the Earth at the same rate. This shows why for a long time people believed that objects fell at different rates. However, if you now put the piece of paper on top of the text book, and drop them together, you will see quite dramatically that the paper and the book drop at precisely the same rate. In the second experiment, clearly the text book, which is less affected by air resistance, is shielding the piece of paper from the affects of air resistance so that both are essentially moving only under the influence of gravity. Consequently, as stated above, they accelerate at the same rate.

Graphs of how the position, velocity, and acceleration of objects under free fall motion change in time follow.

 

Figure 2.3: Free fall acceleration

 

Figure 2.4: Free fall velocity

 

Figure 2.5: Free fall position