MELC4: Explain the subtle distinction between Newton’s 1st Law of Motion (or

Law of Inertia) and Galileo’s assertion that force is not necessary to sustain
horizontal motion

Lesson The Aristotelian and Galilean

1 Conceptions of Vertical, Horizontal
and Projectile Motions
Motion is the action of changing location or position. Life is motion. From the coordination of the
muscles of our body which enables us to walk, run, and dance to the pumping of our hearts to
deliver blood to the different parts of our bodies are motion.

The first evidence of the study of the motion of heavenly bodies can be traced back to the people
of Sumeria and Egypt. While the Greeks were the first ones to study systematically and in detail
the heavenly bodies. They regarded the Earth as the center of the universe, geocentric. This idea
of geocentric earth was replaced by the heliocentric model of Nicolaus Copernicus, where earth
and other planets revolve in circular orbits around the Sun.

These early studies were the foundations of Galileo Galilei, an Italian physicist to revolutionized
Science.

One such philosophical concept held is the philosophy behind motion in two particular points,
namely, the existence of one unifying explanation for the horizontal and vertical motions and how
projectile motions can be derived from the two and the quantification of the “rate of fall” or
“acceleration”.

Aristotle held that the Universe was divided into two regions, the terrestrial region and the celestial
region. He also divided motion into two main classes which are natural motion and violent motion.
On the other hand, Galileo Galilei believed that a projectile motion is a combination of uniform
motion in the horizontal direction and uniformly accelerated motion in the vertical direction.

I. Aristotle’s Concept of Vertical Motion, Horizontal Motion, and

Projectile Motion

1. Vertical Motion
Vertical motion is referred to as natural motion. In a natural motion, the object will move and will
return to its natural state based on the object's material or composition - earth, water, air, and
fire.

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  For example, Aristotle believed that because a ball fell when thrown
upward, its element was earth.

 Smoke goes up the air because it seeks its natural place in the atmosphere.

 Aristotelian concept of natural motion is largely vertical motion which is

falling or rising.

2. Horizontal Motion
An object moving in a violent motion requires push or pull to maintain horizontal motion. Motion
continues only so long as there is an applied force to an object. When the force is removed, motion
stops.

 The example at the right shows piled boxes of food

donations. The boxes will remain on the floor unless
a push or a pull force is applied.

 Violent motion is imposed motion caused by

pushing or pulling.

 Aristotle classified any motion

that required a force as a "violent motion". (He did
not mean violent in the sense that it is marked by
the use of harmful or destructive physical force.)

3. Projectile Motion
Aristotle believed that the projectile motion of an object is parallel to the ground until it is the
object's time to fall back into the ground. An impetus will be kept by the object until such time
that the initial force is forgotten, and the object returns to its natural state to stop moving and fall
to the ground.
He viewed projectile motion as natural and violent motion. He said that heavy objects fall faster
than light ones.

The illustration shows an example of

cannon fired which gives the cannonball
an impetus.

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 A notorious problem for the Aristotelian view was why arrows shot from a bow continued to fly
through the air after they had left the bow and the string was no longer applying force to them.

Aristotle's Explanation:

 Initial motive force transfers to the medium initially surrounding the object a "power"
to act as a motive force.

Air flow

 Medium then forces object into new region, which is then imparted with the "power" to
act.

Power transfer

II. Galileo’s Concept of Vertical Motion, Horizontal Motion, and Projectile Motion
1. Vertical Motion
In the absence of a resistance, objects would fall not depending on their weight, but in the time of
fall. Also, if the object encountered a resistive force from a fluid equal or greater than its weight, it
will slow down and reaches a uniform motion until it reaches the bottom and stops.

 For example, without any

resistance, a 1-kg object will be as
fast as a 10-kg object when falling
because they fall with the same
amount of time, given that they are
released from the same height.

 Also, a stone dropped in the ocean

will sooner or later travel at
constant speed.

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2. Horizontal Motion
An object in motion, if unimpeded, will continue to be in motion, and an external force is not
necessary to maintain the motion. If the Earth’s surface is very flat and extended infinitely, objects
that are pushed will not be impeded. Thus, the objects will continue to move. This kind of motion,
however, is not evident in nature.

For example, if a ball is pushed on an infinitely flat plane, the ball will continue to roll if
unimpeded.

3. Projectile Motion
Galileo performed experiments on uniformly accelerated motion using an inclined plane,
and used the same apparatus to study projectile motion.

 Galileo was credited for quantifying the “rate of fall” by measurement of distance and time
and plotting it graphically. He was able to slow down the “fall” using ramps rather than
viscous materials as Aristotle did resulting to significantly different conclusions related to
the “rate of fall”.

 He correctly measured motion in two independent directions (horizontal and vertical) and
deduced that the “rate of fall” is better measured in terms of downward acceleration.

 He used geometry to provide better description (kinematics) of projectile motion whereby

horizontal motion has zero acceleration (constant speed horizontally) and content vertical
acceleration.

 By varying the ball's horizontal velocity and vertical drop, Galileo was able to determine that
the path of a projectile is parabolic.

 He believed that a projectile is a combination of uniform motion in the horizontal direction

and uniformly accelerated motion in the vertical direction. If it is not impeded, it will
continue to move even without an applied force.

 For example, when you shoot a ball in a basketball ring, the ball does not need a force
to keep it moving.

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 Activity 1: Mini Lab: Aristotle or Galileo?
I. Test the following activities below. Take note that in every activity, both objects should be
dropped at the same time and at the same height. (In choosing the height, be sure that you are
safe).

1. Drop a book and a flat sheet of paper.

2. Drop a book and a sheet of paper crumpled to a ball.
3. Drop a book with a small flat sheet of paper on top of it.

Guide Questions
1. In all three cases, which object reaches the ground first?
2. In activity 1, what causes the flat sheet of paper to move sideways?
3. In activities 2 and 3, why did the objects fall at the same rate regardless of their masses?
4. Whose view is more acceptable? Why?

II. Given two one-peso coins, released at the same time, Coin A is dropped while Coin B is thrown
horizontally coming from the same height.
Guide Question: Which one do you think would reach the ground first? Explain

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 Lesson How Galileo Inferred that Objects
2 in Vacuum fall with Uniform
Acceleration
Motion is defined as the ability of an object to change its position with respect to its
surroundings in given time. Motion is always observed and measured with a point of reference. All
living things show motion whereas non-living things show motion only when force acts on it.
These are the terms associated with motion. First, Free fall, which is an example of motion
with uniform acceleration. Second is acceleration which means the change in velocity with a given
time. Third is speed which is the distance travelled of an object in a specific amount of time.

I. Aristotle's View of Motion

 Aristotle was an Ancient Greek philosophers who
thought that heavier objects fall faster than lighter
ones.
 Force would be needed to have a constant velocity
according to Aristotle.
Force is required for violent but not natural motion.

II. Galileo's View of Motion

 Italian Mathematician. Father of modern Science
 Discredited Aristotelian view of
motion
 Object in vacuum will fall at the same time because in a vacuum there is nothing. Since
there is no air or anything in space, objects that are heavy or light, will fall at the same
time.
 Force is not necessarily required for violent motion.
Force is not required for object moving at constant velocity.

Galileo's Experimentation to prove his ideas about motion:

 Experiment I
Galileo dropped two different weights of cannon ball to the Leaning Tower of Pisa and they fell
at the same time but could not measure the distance travelled therefore he proceeded to another
experimentation.

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  Experiment II

 Galileo used the inclined ramp in order to measure the acceleration of an object. He
used a rolling ball to measure the changes. He measured that objects accelerates at
the same time regardless of their size and mass.

 Speed is the distance traveled by an object divided by the time taken to travel the
distance

 A ball rolling down an inclined plane increases its speed by the same value after every
second

The speed of a rolling ball was found to increase by 2 m/s every second. This means that the
rolling ball would have the following speeds for every given second

 Free Fall and Gravity

 Freely falling bodies undergo constant acceleration in which it is referred to as free fall. The
free fall acceleration represent a symbol g which means gravity.
 At the surface of the earth, gravity (g) is approximately 9.8 m/s2.
 Gravity leads the object to accelerate in a constant rate.

 Horizontal Motion

 View of Aristotle
 Natural state of object at rest
 View of Galileo
 Imagined motion will continue without friction
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 Horizontal versus Vertical Motion

Horizontal: constant in value which represent by x axis

 rolling balls
 inertia
 leaning tower of Pisa

Vertical: changing value which represent by y axis

 free fall
 Aristotle versus Galileo

Activity 2. Construct a Venn Diagram

Point out the similarities and differences between Aristotle's view of motion and
Galileo's view of motion by using a Venn diagram on a separate sheet of paper.

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 Lesson
Understanding Newton’s First Law
3 of Motion
Inertia is the tendency of an object to resist change when in motion or when at rest. Newton’s first
law of motion is also called law of inertia. If an object is moving, inertia will keep it in motion.
When it is at rest, it will continue to be at rest, unless there is a force applied on it

I. Galileo and Inertia

Galileo Galilei 1564-1642

Galileo developed the first concept of laws of motion. Among these are the following:

 Developed the concept of motion in terms of velocity (speed and direction) by

using inclined planes.
 Developed the idea of force that causes motion.
 Determined that the natural state of an object is rest or uniformed motion.
 Developed the idea that objects resist motion, inertia.

According to Galileo, objects in motion eventually stop because of a force called friction. Friction
is a force that opposes motion between any surfaces that are touching. This is based on his
experiment observed in inclined planes. He said that a difference between initial and final heights
was due to the presence of friction. Galileo hypothesized that if friction could be entirely eliminated,
the ball would reach exactly the same height. Or if the ball rolls horizontally and there’s no friction
that acts on it, the ball will never stop.

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II. Isaac Newton and his First Law of Motion
Isaac Newton 1642-1727

Isaac Newton was born the same year

Galileo died. Newton’s law of inertia is
based on Galileo’s idea of inertia. He
expanded Galileo’s work and came up
with his Three Laws of Motion.
Newton’s first law of motion states
that…
An object at rest stays at rest and an
object in motion stays in motion with
the same speed and in the same
direction unless acted upon by an
unbalanced force.

Mass and Inertia

An object’s inertia depends on its mass. Mass is the amount of matter or substance that makes
up an object. It is measured in units called kilogram. An object with a greater mass has a greater
inertia and an object with a lesser mass will also have a lesser inertia.

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 An object’s tendency to resist change in its motion
depends on its mass. Below is a wooden box and a
styrofoam box, which do you think has a greater mass?
lesser? Which has a greater inertia?

Exploring Inertia
Inertia has three types:

 Inertia of rest- an object will stay in place unless something or somebody moves it.
Example: a plate on the table

 Inertia of motion- an object will continue at the same speed until a force acts on it.
Example: a rolling ball

 Inertia of Direction- an object will stay moving in the same direction unless a force acts on
it.
Example: a moving car turning right

Forces
What causes an object to move, to stop or remain at rest? A force causes an object to move,
to stop or remain at rest.

 Force is defined as a push or a pull.

 Newton (N) is the unit of force
 One Newton is the amount of force required to give a 1-kg mass an acceleration of 1
m/s/s. Thus, the following unit equivalency can be stated as:

1 Newton = 1 kg • m/s²
1. Friction

A rolling marble on the floor that suddenly stops when it reached a rough surface does not
stop because of the absence of a force, it stops because of the presence of a force called friction.

 Friction is a force that opposes motion between any surfaces that are
touching.

 Friction occurs because no surface is perfectly smooth.

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  Rough surfaces have more friction than smooth surfaces. Heavier objects
also have more friction because they are pressed harder with greater force than lighter
objects.

 Friction produces heat because it causes the molecules on rubbing

surfaces to move faster and have more energy.

Newton’s first law of motion has always stated that:

An object at rest stays at rest and an object in motion stays in motion with the same speed
and in the same direction unless acted upon by an unbalanced force.

2. Balanced and Unbalanced Forces

A balanced force is a force in which the net force is equal to zero.

What is a net force? A net force is the total combination of forces

(in opposite direction or the same direction) acting on an object.

Let’s consider the free body diagram of a flower vase on the table. There are two forces acting on
the flower vase. The force of the table pushing the flower vase upward also called the normal force
and the gravitational force pushing the flower vase downward.
The forces on the flower vase are balanced
Normal force of the table pushing the flower vase upwards

flower vase

Gravitational force pushing it downwards

Since the two forces are equal and in opposite directions, they balanced each other. The net force
is zero, it means the flower vase is at its equilibrium. There is no unbalanced force acting on it
thus it remains at rest.

What is an unbalanced force?

An unbalanced force is a force in which the net force is greater than zero.

40N 50N

Unbalanced forces cause acceleration. Only unbalanced force can change the motion and direction
of an object.

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 Example: Your pet dog
can move you faster if
he pulls you with
enough force.

Activity 3: Critical Writing

Answer the following questions briefly.

1. State Newton’s first law of motion. What makes the object remain in state of motion
or at rest? Explain why?
2. Explain the difference on Galileo’s idea of inertia and Newton’s first law of motion.

Performance Task: Create an Acrostic Poem

Now that you have understood the law of inertia, create an acrostic poem using the word INERTIA.
In creating your poem, you may share what you have learned or experienced in daily life that
shows INERTIA.
I
N
E
R
T
I
A

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