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object moving across Earth’s surface requires a continuously
EXAMPLE 2.5 applied force to continue moving. These ideas were based
An automobile moving at 25.0 m/s comes to a stop in 10.0 s when the on observing and thinking, not measurement, and no one
driver slams on the brakes. How far did the car travel while stopping? checked to see if they were correct. It would take about two
thousand years before people began to correctly understand
SOLUTION motion.
Aristotle did recognize an association between force and
The car has an initial velocity of 25.0 m/s (v i ) and the final velocity of
motion, and this much was acceptable. It is partly correct be-
0 m/s (v f ) is implied. The time of 10.0 s (t) is given. The problem asked
cause a force is closely associated with any change of motion,
for the distance (d). The relationship given between v, t, and d is given
in equation 2.1, v = d/t, which can be solved for d. The average velocity as you will see. This section introduces the concept of a force,
(v), however, is not given but can be found from equation 2.3. which will be developed more fully when the relationship be-
tween forces and motion is considered.
_ d _
v f + v i

v = v = ∴ d = v · t A force is a push or a pull that is acting on an object. Con-

2 t
sider, for example, the movement of a ship from the pushing
v i = 25.0 m/s − _
v f + v i

Since v = of two tugboats (Figure 2.7). Tugboats can vary the strength
,
v f = 0 m/s 2
of the force exerted on a ship, but they can also push in differ-
t = 10.0 s ( _ ) ent directions. What effect does direction have on two forces
v f + v i

−

v = ? you can substitute 2 for v, and acting on an object? If the tugboats are side by side, pushing
(
)
d = ? _ in the same direction, the overall force is the sum of the two

v f + v i

d = 2 (t) forces. If they act in exactly opposite directions, one pushing on
m _ m _ each side of the ship, the overall force is the difference between
)

0 + 25.0

s
s
= ( __ (10.0 s) the strength of the two forces. If they have the same strength,

2
the overall effect is to cancel each other without producing any
m _
= 12.5 × 10.0 × s motion. The net force is the sum of all the forces acting on

s
an object. Net force means “final,” after the forces are added
_
m·s
= 125 (Figure 2.8).
s
= 125 m When two parallel forces act in the same direction, they can
be simply added. In this case, there is a net force that is equiva-
lent to the sum of the two forces. When two parallel forces act
EXAMPLE 2.6 in opposite directions, the net force is the difference in the
direction of the larger force. When two forces act neither in a
What was the deceleration of the automobile in example 2.5?
2
(Answer: –2.50 m/s ) way that is exactly together nor exactly opposite each other, the
result will be like a new, different net force having a new direc-
tion and strength.
Forces have a strength and direction that can be repre-
sented by force arrows. The tail of the arrow is placed on the
CONCEPTS Applied object that feels the force, and the arrowhead points in the di-
rection in which the force is exerted. The length of the arrow is
Acceleration Patterns proportional to the strength of the force. The use of force arrows
helps you visualize and understand all the forces and how they
Suppose the radiator in your car has a leak and drops of
contribute to the net force.
fluid fall constantly, one every second. What pattern will
There are four fundamental forces that cannot be ex-
the drops make on the pavement when you accelerate
the car from a stoplight? What pattern will they make plained in terms of any other force. They are the gravita-
when you drive at a constant speed? What pattern will you tional, electromagnetic, weak, and strong nuclear forces.
observe as the car comes to a stop? Use a marker to make Gravitational forces act between all objects in the universe—
dots on a sheet of paper that illustrate (1) acceleration, between you and Earth, between Earth and the Sun, between
(2) constant speed, and (3) negative acceleration. Use the planets in the solar systems—and, in fact, hold stars in
words to describe the acceleration in each situation. large groups called galaxies. Switching scales from the very
large galaxy to inside an atom, we find electromagnetic forces
acting between electrically charged parts of atoms, such as
electrons and protons. Electromagnetic forces are responsible
FORCES for the structure of atoms, chemical change, and electricity
The Greek philosopher Aristotle considered some of the and magnetism. Weak and strong forces act inside the nucleus
first ideas about the causes of motion back in the fourth cen- of an atom, so they are not as easily observed at work as are
tury b.c. However, he had it all wrong when he reportedly gravitational and electromagnetic forces. The weak force is
stated that a dropped object falls at a constant speed that is involved in certain nuclear reactions. The strong nuclear force
determined by its weight. He also incorrectly thought that an is involved in close-range holding of the nucleus together. In
32 CHAPTER 2 Motion 2-8