S E C T I O N 2 . 6 • Freely Falling Objects
41
are released. Any freely falling object experiences an acceleration directed
downward, regardless of its initial motion.
We shall denote the magnitude of the free-fall acceleration by the symbol g. The value of
g near the Earth’s surface decreases with increasing altitude. Furthermore, slight varia-
tions in g occur with changes in latitude. It is common to define “up” as the % y direction
and to use y as the position variable in the kinematic equations. At the Earth’s surface,
the value of g is approximately 9.80 m/s
2
. Unless stated otherwise, we shall use this value
for g when performing calculations. For making quick estimates, use g ! 10 m/s
2
.
If we neglect air resistance and assume that the free-fall acceleration does not vary
with altitude over short vertical distances, then the motion of a freely falling object mov-
ing vertically is equivalent to motion in one dimension under constant acceleration.
Therefore, the equations developed in Section 2.5 for objects moving with constant accel-
eration can be applied. The only modification that we need to make in these equations
for freely falling objects is to note that the motion is in the vertical direction (the y direc-
tion) rather than in the horizontal direction (x) and that the acceleration is downward
and has a magnitude of 9.80 m/s
2
. Thus, we always choose a
y
! #
g ! # 9.80 m/s
2
, where
the negative sign means that the acceleration of a freely falling object is downward. In
Chapter 13 we shall study how to deal with variations in g with altitude.
Quick Quiz 2.6
A ball is thrown upward. While the ball is in free fall, does its
acceleration (a) increase (b) decrease (c) increase and then decrease (d) decrease and
then increase (e) remain constant?
Quick Quiz 2.7
After a ball is thrown upward and is in the air, its speed
(a) increases (b) decreases (c) increases and then decreases (d) decreases and then
increases (e) remains the same.
Conceptual Example 2.9 The Daring Sky Divers
A sky diver jumps out of a hovering helicopter. A few seconds
later, another sky diver jumps out, and they both fall along the
same vertical line. Ignore air resistance, so that both sky divers
fall with the same acceleration. Does the difference in their
speeds stay the same throughout the fall? Does the vertical dis-
tance between them stay the same throughout the fall?
Solution At any given instant, the speeds of the divers are
different because one had a head start. In any time interval
$
t after this instant, however, the two divers increase their
speeds by the same amount because they have the same ac-
celeration. Thus, the difference in their speeds remains the
same throughout the fall.
The first jumper always has a greater speed than the sec-
ond. Thus, in a given time interval, the first diver covers a
greater distance than the second. Consequently, the separa-
tion distance between them increases.
Example 2.10 Describing the Motion of a Tossed Ball
A ball is tossed straight up at 25 m/s. Estimate its velocity at
1-s intervals.
Solution Let us choose the upward direction to be positive.
Regardless of whether the ball is moving upward or down-
ward, its vertical velocity changes by approximately # 10 m/s
for every second it remains in the air. It starts out at 25 m/s.
After 1 s has elapsed, it is still moving upward but at 15 m/s
because its acceleration is downward (downward accelera-
tion causes its velocity to decrease). After another second, its
upward velocity has dropped to 5 m/s. Now comes the tricky
part—after another half second, its velocity is zero. The
ball has gone as high as it will go. After the last half of this
1-s interval, the ball is moving at # 5 m/s. (The negative sign
tells us that the ball is now moving in the negative direction,
that is, downward. Its velocity has changed from % 5 m/s to
#
5 m/s during that 1-s interval. The change in velocity is
still # 5 m/s # (% 5 m/s) ! # 10 m/s in that second.) It
continues downward, and after another 1 s has elapsed, it is
falling at a velocity of # 15 m/s. Finally, after another 1 s, it
has reached its original starting point and is moving down-
ward at # 25 m/s.
▲
PITFALL PREVENTION
2.7 The Sign of g
Keep in mind that g is a positive
number—it is tempting to substi-
tute # 9.80 m/s
2
for g, but resist
the temptation. Downward gravi-
tational acceleration is indicated
explicitly by stating the accelera-
tion as a
y
! #
g.
▲
PITFALL PREVENTION
2.8 Acceleration at the
Top of The Motion
It is a common misconception
that the acceleration of a projec-
tile at the top of its trajectory
is zero. While the velocity at the
top of the motion of an object
thrown upward momentarily goes
to zero, the acceleration is still that
due to gravity at this point. If the
velocity and acceleration were
both zero, the projectile would
stay at the top!