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PHYSICAL SCIENCE
Quarter II – Week 2
Aristotelian and Galilean
Views of Motion
CONTEXTUALIZED LEARNING ACTIVITY SHEETS
SCHOOLS DIVISION OF PUERTO PRINCESA CITY
Horizontal Motion
Aristotle recognized an association between force and motion. It is partly correct because a force
is closely associated with any change of motion. But he was wrong when he thought that an object moving
across Earth’s surface requires a continuously applied force to continue moving. You might agree with
Aristotle because everyday experience seems to indicate that his idea is correct. After all, moving objects
that are not pushed or pulled do come to rest. It would seem that an object keeps moving only if a force
continues to push it. But thousand years later, Galileo discredited the Aristotelian view.
Galileo proposed that objects moving at a constant velocity require no force to keep them moving,
unless acted upon by friction. Galileo was one of the first to recognize the role of friction in opposing
motion. His experiment with marbles (see figure 1) illustrates his theory. Image (a) shows that when a
marble is released at any height of an inclined plane, it will roll down and climb another inclined plane,
reaching a lower height because the opposing friction is impeding its climb. In smoother plane (b) the initial
height of marble upon its release has no significant difference to its final height on the other side of the
inclined plane. It can be concluded that if friction is absent, there will be no force counteracting the initial
velocity and there will be no change in motion. In (c), the inclined plane is also smooth but the other side
is more tilted. The result is the same with (b). Galileo concluded that the object would move forever with
constant velocity as long as there was no unbalanced force acting to change the motion.
Figure 1. Illustration of Galileo’s thought experiment
Vertical Motion
Falling objects such as rain and snow, leaves and twigs portray a vertical motion. Aristotle
reportedly stated that an object falls at a uniform speed that is proportional to its weight. This means that
heavier objects would fall faster than lighter objects, and that its initial speed is the same with its final
speed when it hits the ground. Sounds true, right? But he had it all wrong.
In a popular story Galileo contradicted the views of Aristotle by dropping a solid iron ball and a
solid wooden ball simultaneously in the Leaning Tower of Pisa. The balls hit the ground at the same time
according to the story. This shows that weight do not determine the velocity of a falling object. But in daily
life, why do we observe that heavier twigs fall faster than the lighter dried leaves? It seems that Galileo is
wrong. This is explainable by air resistance. According to Galileo, if air resistance is removed light and
heavy objects fall at the same time. Vertical motion without air resistance is called free fall. Free fall
considers only gravity and neglects air resistance. Soon after Galileo, the air pump that is used to remove
air from a glass tube was invented. This space inside the tube is called vacuum. In vacuum coin and feather
fall together.
Galileo also observed that the velocity of an object in free fall
increased as the object fell. He made calculations involving distance,
velocity, and time and was the first to introduce the concept of acceleration.
According to his calculations, due to gravity, freely falling objects should
cover a distance proportional to the square of the time of the fall (d ∞ t 2). In
other words, the object should fall 4 times as far as in 1s (2 2 = 4), 9 times as
far in 3s (32=9), and so on. Galileo checked his calculation by rolling balls on
an inclined board with a smooth groove in it. He found as predicted that the
falling object accelerates towards Earth’s Figure 2. Free fall acceleration surface at
a constant rate (see figure 2).
(Source: Bill W. Tillery, Physical Science, New York: MacGraw-Hill, 2012, 26, 34-37.)
Projectile Motion
Any thrown objects are called projectile. Aristotle explained what maintains the motion of a
projectile after it is thrown through the concept of antiperistasis. See illustration in Figure 3.
Figure 3. Antiperistasis
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Subject to antiperistasis, a projectile must come in contact with a medium. Medium is the required
contact force such as air. In figure 3-a, the motive force (push from the bow) given to the arrow will be
transferred to the medium (in this case, air). The medium then fills in the space left behind by the arrow
and provides the necessary push for the object to
continue moving.
Impetus is impressed
Later Aristotelian Developments
John Philoponus (~490-570 CE) argued that the
medium resists the motion, contradictory to Aristotle’s
idea that it aid the motion of an object. Thus, the motive
force is not transmitted to the medium but retained in the
object itself. Jean Buridan (1300—1358) supported
Philoponus’ idea by adding the concept of impetus.
force in projectile t
cause of cont
motion. The
initia
amo
Figure 4. Jean Buridan
Figure 5. a page from one of Galileo's
manuscripts in which he writes down the
figures he obtained in performing this
experiment himself.
Later, simply by more careful observation it was realized that
projectiles actually follow some sort of a curved path. No one
knew what sort of curve it was until Galileo. He reasoned that a
projectile shot from a cannon is not influenced by only one
motion, but by two – vertical and horizontal. The motion that acts
vertically is the force of gravity, and this pulls the projectile down
as shown in figure 5-line BN (from point B to point N). But while
gravity is pulling the object down, the projectile is also moving
forward, horizontally at the same time as shown in figure 5-line
BCDE. And this horizontal motion is uniform and
constant according to his principle of inertia. What he
Figure 5. Galileo's projectile motion actually comes to see is that, in
fact, the curve as shown in figure 5-BIFH has an exact
mathematical shape called the parabola.
(Source: “Galileo and Aristotle on Motion”, mcm.edu, Accessed January 20,2021, http://www.mcm.edu/academic/galileo/ars/arshtml/mathofmotion2.html)
Aristotle and Galileo both have made important contribution towards our understanding in the world
of Physics. Both of them have conflicting ideas specifically their views of motion. Aristotle’s views on motion
were based on the natural state of the object and based on his observation and thinking, without measurements
and mathematical verification while Galileo’s views on motion were more quantitative and based on
experiments.
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ANSWER SHEET
WEEK 3
Aristotelian and Galilean Views of Motion
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ASSESSMENT
Directions: Read and analyze each question carefully. Write your answer on a sheet of paper.
1. Which of the following does Galileo believe about motion?
A. A violent motion is caused by an external force.
B. Objects move because of their elemental composition.
C. Objects will continue to be in uniform motion if unimpeded.
D. Heavier objects fall faster than lighter objects if there is no resistance.
2. Which of the following agrees with Galileo’s view of motion?
A. Smoke rises because the sky is its natural place.
B. Planets revolve around the Sun because of a Prime Mover.
C. If a 10-kg boulder is dropped to the sea, it will stop midway.
D. A feather will be as fast as an iron ball if dropped in a vacuum.
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3. Which of the following is true about an arrow when shot?
A. It rises because arrows are mostly air.
B. It has both uniform motion and uniformly accelerating motion.
C. It creates a vacuum that sucks air in, and the air pushes the arrow.
D. It has a continuous applied force from the person who shot the arrow.
4. Which of the following agrees with Galileo’s view of motion?
A. An iron ball will always fall faster than a rubber ball.
B balloon rises because it is mostly air, and the sky is its natural place.
C. A box will stop sliding in a frictionless plane if the applied force is removed.
D. A flying baseball has both uniform motion and uniformly accelerating motion.
5. According to Galileo's view, which will reach the floor of the vacuum chamber first, a sheet of paper or a
bowling ball?
A. They will reach the floor at the same time.
B. The bowling ball will reach the vacuum chamber.
C. The sheet of paper will reach the vacuum chamber.
D. They will not reach the floor and will be sucked in the vacuum.
PERFORMANCE TASK #2
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