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FIGURE 2.16 A horizontal projectile has the same horizontal velocity throughout the fall as it accelerates toward the surface, with the
combined effect resulting in a curved path. Neglecting air resistance, an arrow shot horizontally will strike the ground at the same time as one
dropped from the same height above the ground, as shown here by the increasing vertical velocity arrows.

2.6 THREE LAWS OF MOTION
In the previous sections, you learned how to describe motion in
terms of distance, time, velocity, and acceleration. In addition,
you learned about different kinds of motion, such as straight-
line motion, the motion of falling objects, and the compound
motion of objects projected up from the surface of Earth. You
were also introduced, in general, to two concepts closely associ-
ated with motion: (1) that objects have inertia, a tendency to
resist a change in motion, and (2) that forces are involved in
a change of motion.
The relationship between forces and a change of motion
FIGURE 2.17 A football is thrown at some angle to the is obvious in many everyday situations (Figure 2.18). When a
horizon when it is passed downfield. Neglecting air resistance, the car, bus, or plane starts moving, you feel a force on your back.
horizontal velocity is a constant, and the vertical velocity decreases, Likewise, you feel a force on the bottoms of your feet when an
then increases, just as in the case of a vertical projectile. The elevator starts moving upward. On the other hand, you seem
combined motion produces a parabolic path. Contrary to statements
to be forced toward the dashboard if a car stops quickly, and
by sportscasters about the abilities of certain professional
quarterbacks, it is impossible to throw a football with a “flat it feels as if the floor pulls away from your feet when an eleva-
trajectory” because it begins to accelerate toward the surface as tor drops rapidly. These examples all involve patterns between
soon as it leaves the quarterback’s hand. forces and motion, patterns that can be quantified, conceptualized,

40 CHAPTER 2 Motion 2-16