2 Kinematics - BC Science Physics 11

2.4 Acceleration of Bodies Due to Gravity
Warm Up
Take a piece of paper and book in each hand and raise them to about the height of your shoulder. Predict which
one will hit the ground first if they are both dropped at the same time. Drop both and record results. Now
crumple the same piece of paper and repeat. Describe and suggest reasons for any differences between the two
events.
Free Fall
Figure 2.4.1 These sky divers are in free fall until they
open their parachutes to slow their descent.
One of the most common situations involving uniform acceleration is the phenomenon
known as free fall. For example, if a coin drops out of your pocket, it accelerates toward
the ground. If the effects of air resistance are ignored, the acceleration of the coin toward
the ground is uniform. The coin starts its downward fall with zero speed, but gains
speed as it falls toward Earth. Since gravity is the cause of the acceleration, we call the
acceleration during free fall the acceleration of gravity. The acceleration of gravity is
given a special symbol, g.
The magnitude of g depends on your location. At Earth’s
surface, g is approximately 9.81 m/s 2 . At higher altitudes,
g decreases. For our present purposes, g is assumed to be
constant at Earth’s surface and to be 9.81 m/s 2 . On the Moon,
the magnitude of g is approximately 1/6 of what it is here on
Earth’s surface. A body in free fall near the Moon’s surface has
an acceleration of gravity of only 1.60 m/s 2 .
Of course, the four equations for uniform acceleration
apply to free fall as well as other uniform acceleration
situations. The symbol g may be substituted for a in those
equations.
64 Chapter 2 Kinematics © Edvantage Interactive 2012 ISBN 978-0-9864778-3-6