1
INERTIAL REFERENCE FRAMES,
ACTION-REACTION PAIRS, AND FREEBODY DIAGRAMS
for GENERAL PHYSICS 1/ Grade 12
Quarter 1/ Week 4
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FOREWORD
This self-learning kit will serve as guide on knowing concepts about
Inertial Frame of Reference, Action-Reaction Pairs and Free-body Diagrams.
In this kit, we will be tackling what Inertial Frame of Reference really is
and how it plays a vital role in concretizing Newton’s First Law. In addition, you
will know how to draw free-body diagrams. It will be your aid as you learn new
ideas and enrich your existing knowledge about Physics concepts. Lessons on
identifying action-reaction pairs are also focused. This lesson will bring us to
more concrete understanding and appreciation Newton’s 3rd Law and of the
principles and concepts it governs. This will also pave way to a deeper curiosity
and interest in its application in our daily life experiences. We already know
that physics is concerned with the motion of objects and the quantitative
analysis of that motion. To properly define such motion is not an easy thing. It
has a lot of ambiguity especially when dealing with interacting motions. Thus,
learning Newton’s third law along with the first and second is equally of great
importance. In this particular SLK, we will be tackling about how Action –
reaction pair really is and how it plays a vital role in concretizing Newton’s Law
of Interaction.
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OBJECTIVES
At the end of the lesson, the students are expected to:
K: define inertial frame of reference;
: identify action-reaction pairs;
S: draw free-body diagrams; and
A: appreciate the application of action-reaction pairs to real-life setting.
LEARNING COMPETENCIES
Define inertial frame of reference (STEM_GP12N-Id-28).
Identify action-reaction pairs (STEM_GP12N-Id31).
Draw free-body diagrams (STEM_GP12N-Id32).
I.
WHAT HAPPENED
PRE-TEST:
TRUE OR FALSE: Write the word “true” if the statement is correct, and “false” if
otherwise. Write your answers in your notebook/Answer Sheet.
____________ 1. Newton’s First Law states that an object at rest will suddenly
move even if there is no force acting upon it.
____________ 2. Inertial Frame of Reference should be a moving object.
____________ 3. Earth may not be considered as inertial frame of reference with
respect to its rotation and revolution.
______ 4. If you are riding in a boat, the water current should be considered
as inertial frame of reference instead of the under- water sand.
____________ 5. Newton’s Law of interaction states that in every force applied
to an object, there is an equal and opposite reaction.
____________ 6. When you push the wall, the force of the wall acting back into
your hand is the action force.
____________ 7. If the force applied to an object is 20N, the object applies
exactly 20N back.
___________ 8. A ball that bounces after thrown downward; the floor is an
example of opposite reaction.
____________ 9. If a metal ball falling on the ground that does not bounce, it
means that in the scenario, no reaction force is applied.
____________ 10. In a vector diagram, the size of the force arrow reflects the
direction of the force.
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II.
WHAT I NEED TO KNOW
We already know that in order to fully grasp the
concept of Inertial Frame of Reference, we need to
understand that it is a subtopic in motion. Motion is an
excellent topic to begin our study of Physics. We see motion
in the activities people do every day: walking down the
stairs, jogging, swimming, driving a motorcycle or riding a
bus. Motion can also be observed in nature: river flowing,
raindrops falling, clouds moving or winds blowing. This would
also help us appreciate how these things are going on and
even infer what might happen next in reference to the
currently witnessed motions.
The study of motion is divided into kinematics and
dynamics. Kinematics is a quantitative description of
motion without reference to its physical causes. Here, we
define terms like displacement, velocity, and acceleration.
Dynamics is the study of the relationship between motion
and force.
“Knowledge is Power”. As we go along with the
lessons, let us be guided that this saying can be made
possible as we put into practice the things we have
learned.
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DISCUSSION
Inertial Frame of Reference
The notion on frame of reference is always associated with the concept
of position. The term position refers to the location of the object with respect to
some reference frame. A reference frame is a physical entity such as ground,
room, or a building to which motion or position of an object is being referred.
For instance, we say that the gymnasium is 20 meters to the right of the main
gate. Our point of reference in this example is the gate.
Suppose you are standing on a skateboard while in the top of a bus that
is traveling on a straight road and speeding up. If you could stand on the
skateboard, you would start moving backward relative to the bus as it gains
speed. If instead the bus was slowing to a stop, you would start moving forward
down the roof. In either case it looks as though Newton’s First Law is not
obeyed; there is no net force acting on you, yet your velocity changes. What
do you think is the reason?
The point is that the bus is accelerating with respect to the earth and is
not a suitable frame of reference for Newton’s first law. This law was valid in
some frames of reference and not valid in others. A frame of reference in which
Newton’s first law is valid is called an inertial frame of reference. The earth is at
least approximately an inertial frame of reference, but bus is not. (The earth is
not a completely inertial frame, owing to the acceleration of motion associated
with its rotation and its motion around the sun. These effects are quite small,
however; because Newton’s first law is used to define what we mean by an
inertial frame of reference. It is sometimes called law of inertia. One thing that
we have to consider is that an inertial reference should be a non-moving
basis/reference of the on-going motions around it or within it.
Figure 1. Standing on a skateboard while in the top of a bus that is traveling on a straight
road and speeding up
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Figure 2. An illustration showing the skate boarder on top of the bus (A), the moving bus (B),
and the road (C)
To disentangle few confusions, let us first recall Newton’s first law, the”
Law of Inertia.” It states that an object at rest will remain at rest unless an external
force is acted upon it, or an object in motion will remain in motion unless an
external force is acted upon it. Now let’s take a look at the given scenario:
The skate boarder who is top of the bus, moves backwards while the bus
speeds up but if we notice, there was neither a pull nor a push being made for
it to move backwards. In reference to the bus, the skate boarder is really
moving back, but in reference to the ground it seemed that the skate boarder
is unmoved or even considering the fact it is slightly moved in reference to the
ground. These two references of motion, the ground and the bus, will lead us
to mind which among the two we should consider as basis of motion.
A frame of reference that moves with constant velocity with respect to
an inertial frame is also an inertial frame. For example: Newton's laws work in
the physics lab, which is fixed to the Earth. Therefore, they also work in a train
moving with constant velocity with respect to the Earth.
An inertial frame of reference is a reference frame, i.e., point of view,
where the property of inertia holds true. A non-inertial frame of reference is the
reference frame where the property of inertia does not seem to hold true. This
fictional force is not actually a force at all but an effect of inertia.
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Action-Reaction Pairs
Action-Reaction pair applies to almost all part of our lives which involve
force. Newton’s law of Interaction explains why an egg breaks after hitting an
edge of the table, why cue ball in a billiard bounce back after hitting the sides
of the billiard table, and why a car suddenly stops after bumping with a steady
vehicle in front of it. With such understanding on this realty, they may be able to
adjust or control the amount of force in their daily activities which involves
pushing or pulling such as in many different sports or work activities.
Suppose that you are trying to hit the wall with the button of a push pen.
Note that the wall is immovable. In this case, it is the pen, which is trying to
make a push, but obviously the push button of the push-pen is pushed inside
and thus contracted. This evidence is an illustration that there is an opposite
pushing force from the wall acting back to the pen and thus perfectly
demonstrates action reaction pairs.
Newton’s third Law as it says, “in every force applied to an object, there
is an equal opposite force acting back on it”. In this particular Interaction, the
push made by the pen against the wall is the action, while the action push
made by the wall to the pen is the reaction.
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Newton’s Third Law of Motion
Statement: In every action applied, there is an equal and opposite
force/reaction acting back.
This means that if a body A exerts a force F1 on a body B, and the body
B exerts a force F2 on body A, then:
F1=F2
That is, they are equal in magnitude but opposite in direction.
Whenever two bodies interact, they will exert equal and opposite forces
on each other.
This law can be observed in every around us. Below are some of the
action reaction pairs:
1) A man walking on the ground: While walking, a person pushes the
ground in the backward direction, and the ground in return pushes
the person in the forward direction, thus making him/her to walk.
2) When a person is sitting on a chair, his weight is opposed by a
reaction force from the chair (assuming it must be there because of
Newton’s third law of Motion). In turn, both forces cancel each other
out, and the person enjoys the equilibrium position.
There are a million more examples of Newton’s third law of motion in
our environment. If you observe closely, you can find it anywhere and
everywhere.
Example 1:
An object of weight 45 N rests on the table. (a) What are the forces
acting
on the object? (b) What are the reactions to these forces?
Solution:
a. Forces acting on an object are its weight and the upward force by the
table.
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b. Weight is the downward pull of the Earth on the object. Weight is 45N.
The reaction to this force is an upward force of 45 N exerted by the
object on Earth.
Free-Body Diagrams
Free-body diagrams are diagrams used to show the relative magnitude
and direction of all forces acting upon an object in a given situation. The size
of the arrow in a free-body diagram reflects the magnitude of the force. The
direction of the arrow shows the direction that the force is acting. Each force
arrow in the diagram is labelled to indicate the exact type of force. It is
generally customary in a free-body diagram to represent the object by a box
and to draw the force arrow from the center of the box outward in the
direction that the force is acting.
Example 2:
A person standing on the ground.
(The free-body diagram)
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Example 3:
A guava is free falling from its branch. Ignore air resistance.
(The free-body diagram)
Example 4:
An easterly wind is blowing ball cotton.
(The free-body diagram)
Example 5:
A student applied a leftward force to a chair in order to move it across
the floor at constant velocity. Considering frictional forces and neglecting air
resistance, the free-body diagram is shown below.
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Steps on How to Draw Free-body Diagrams
Step 1: Know the various types of forces.
1. Contact Forces:
2. Action at a Distance Forces:
Symbol
Ffric - Frictional Force
Ftens - Tension Force
Fnorm - Normal Force
Fair - Air Resistance Force
Fapp - Applied Force
Fspring - Spring Force
Fgrav - Gravitational Force
Felect - Magnetic Force
Step 2: Determine the direction in which each force is acting.
Step 3: Draw a box and add arrows for each existing force in the
appropriate direction.
Step 4: Label each force arrow according to its type.
Example 6:
A book is at rest on a tabletop. Diagram the forces acting on the book.
Step 1: The types of forces present are Gravitational Force for the weight
of the book and Normal Force for the surface exerting an upward force
upon the book in order to support the weight of the book.
Step 2: The direction of Gravitational Force is downward while the
Normal Force is Upward.
Step 3:
Step 4: The complete Free-Body diagram is shown below.
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Example 7:
A senior high school learner rests a backpack upon his shoulder. The
pack is suspended motionless by one strap from one shoulder. Draw a freebody diagram for this situation.
Step 1: The types of forces present are the Gravitational Force for the
weight of the back-pack and Tension Force exerted by a strap when it
is pulled tight by the weight of the pack on opposite ends.
Step 2: The direction of the Gravitational force is downward while
upward direction for Tension Force.
Step 3.
Step 4. The complete Free-Body diagram is shown below:
Example 8:
A table is pushed across the floor.
Step 1: There are 4 forces acting on a table:
1. The gravitational force, weight exerted by the earth on the
table;
2. The normal force exerted by the floor on the table;
3. The applied force to push the table; and
4. The frictional force exerted by the floor on the block in the
Direction opposite the motion.
Step 2: The directions of the forces:
Gravitational force – downward
Normal force – upward
Applied force – rightward
Frictional force – leftward
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Step 3:
Step 4. Free-body diagram
Example 9:
A box on an inclined plane 300 with the level ground with an acting force
pulls the box upward and friction is considered. Draw the free-body diagram.
Step 1: There are 4 forces acting on a box:
1. The gravitational force for the weight of the box;
2. The normal force exerted by the inclined plane on a box;
3. The applied force to pull the box;
4. The frictional force exerted by the inclined plane on the box in
direction opposite the motion due to the applied force.
Step 2: The directions of the forces:
1. Gravitational force - downward (perpendicular with the level
ground)
2. Normal force – upward (perpendicular with the inclined)
3. Applied force – leftward (300 with the horizontal)
4. Frictional force – rightward (300 with the horizontal)
Step 3:
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Step 4: The free-body diagram
Performance Task:
Directions: Draw the free-body diagram of the following situations. Use
the 4-step method. Do this in your Science notebook/Answer Sheet.
•
•
•
•
•
RUBRICS
Answers with correct list of forces, directions, and
diagram with correct label of force symbols
Answers with correct list of forces, directions, and
diagram with correct label of force symbols
Answers with correct list of forces, directions, and
diagram with correct label of force symbols
Diagram only without force symbols
With answers but incorrect
10 points
8 points
7 points
6 points
5 points
Situations:
1. A dead branch free-falling from a tree. Neglect air resistance.
2. A glider gliding from the top of a mountain to the ground at constant
velocity.
3. The girl is pushing a plastic container to the right across the floor.
4. A boy is sitting on a swing, is suspended motionless. (His feet are not
touching the ground). The swing seat is supported by two ropes
attached to the branch of a tree.
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III.
WHAT I HAVE LEARNED
EVALUATION/POST-TEST
I.
TRUE OR FALSE: Write the word “true” if the statement is correct, and
“false” if otherwise. Write your answers in your notebook/Answer Sheet.
___________ 1. When you push the wall, the force of the wall acting back into
your hand is the action force.
___________ 2. If the force applied to an object is 20N, the object applies
exactly 20N back.
__________ 3. A ball that bounces after thrown downward; the floor is an
example of opposite reaction.
___________ 4. If a metal ball falling on the ground that does not bounce, it
means that in the scenario, no reaction force is applied.
___________ 5. In a vector diagram, the size of the force arrow reflects the
direction of the force.
___________ 6. Newton’s First Law states that an object at rest will suddenly
move even if there is no force acting upon it.
___________ 7. Inertial Frame of Reference should be a moving object.
______ 8. Earth may not be considered as inertial frame of reference with
respect to its rotation and revolution.
____ 9. If you are riding in a boat, the water current should be considered
as inertial frame of reference instead of the under- water sand.
___________10. Newton’s Law of interaction states that in every force applied
to an object, there is an equal and opposite reaction.
II.
IDENTIFYING ACTION AND REACTION: Write “A” if the force being
referred to is the action force and “R” if the force being referred to is a
reaction force. Write your answers on your Science notebook/Answer
Sheet.
_____1. The force from of the ground to a man’s shoes as he walks
_____2. The force of the feet towards the kicked soccer ball
_____3. The force of the punching bag to the gloves of a boxer
_____4. The force from the car hitting a post in an accident
_____5. The force of the water to the boat as the boat propels
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REFERENCES
Free Body Diagrams, Tutorials with Examples and Explanations. Retrieved
from
https://www.problemsphysics.com/forces/free_body_diagram_forces.
html
Newton's Laws of Motion Tutorial.
Retrieved from https://www.physicsclassroom.com/CLASS/newtlaws
PADUA, ALICIA L., CRISOSTOMO, M. RICARDO: Practical and
Explorational Physics (Modular Approach) Copyright 2003, page 75 and
88.
Siverio, Angelina A., et. al.: PHYSICS, Exploring Life Through Science:
PHOENIX PUBLISHING HOUSE: Page 50 – 51.
SILVERIO, ANGELINA A., GLORIA DECASTRO-BERNAS, PhD. PHYSICS
Exploring Life Through Science SECOND EDITION: 927 Quezon Avenue,
Quezon City, PHOENIX PUBLISHING HOUSE INC. Copyright 2012, page
90.
Young, Hugh D., et. al.: PEARSON EDUCATION SOUTH ASIA PTE. LTD.,
UNIVERSITY PHYSICS WITH MODERN PHYSICS: Page 113 – 114.
Young, Hugh D., Freedman, Roger A., and Lewis Ford.: PEARSON
EDUCATION SOUTH ASIA PTE. LTD.: UNIVERSITY PHYSICS WITH MODERN
PHYSICS: Page 123 – 126.
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DEPARTMENT OF EDUCATION
SCHOOLS DIVISION OF NEGROS ORIENTAL
SENEN PRISCILLO P. PAULIN, CESO V
Schools Division Superintendent
JOELYZA M. ARCILLA EdD
OIC - Assistant Schools Division Superintendent
MARCELO K. PALISPIS EdD JD
OIC - Assistant Schools Division Superintendent
NILITA L. RAGAY EdD
OIC - Assistant Schools Division Superintendent/CID Chief
ROSELA R. ABIERA
Education Program Supervisor – (LRMDS)
ARNOLD R. JUNGCO
PSDS-Division Science Coordinator
MARICEL S. RASID
Librarian II (LRMDS)
ELMAR L. CABRERA
PDO II (LRMDS)
DEXTER D. PAIRA
RONALD G. TOLENTINO
Writers
ROSEWIN P. ROCERO
Illustrator/Lay-out Artist
_________________________________________
ALPHA QA TEAM
LIEZEL A. AGOR
EUFRATES G. ANSOK JR.
JOAN Y. BUBULI
MA. OFELIA I. BUSCATO
DEXTER D. PAIRA
LIELIN A. DE LA ZERNA
BETA QA TEAM
ZENAIDA A. ACADEMIA
ALLAN Z. ALBERTO
EUFRATES G. ANSOK JR.
DORIN FAYE D. CADAYDAY
MERCY G. DAGOY
ROWENA R. DINOKOT
RANJEL D. ESTIMAR
MARIA SALOME B. GOMEZ
JUSTIN PAUL ARSENIO C. KINAMOT
LESTER C. PABALINAS
ARJIE T. PALUMPA
DISCLAIMER
The information, activities and assessments used in this material are designed to provide accessible
learning modality to the teachers and learners of the Division of Negros Oriental. The contents of this module
are carefully researched, chosen, and evaluated to comply with the set learning competencies. The writers
and evaluator were clearly instructed to give credits to information and illustrations used to substantiate this
material. All content is subject to copyright and may not be reproduced in any form without expressed written
consent from the division.
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SYNOPSIS AND ABOUT THE AUTHORS
This self-learning kit contains concepts on
inertial frame of reference, action-reaction pairs and
free-body diagrams.
A reference frame is a physical entity such as
ground, room, or a building to which motion or position
of an object is being referred. A frame of reference in
which Newton’s first law is valid is called an inertial
frame of reference. It is a reference frame, i.e., point
of view, where the property of inertia holds true.
On the other hand, Newton’s third Law as it
says, “in every force applied to an object, there is an
equal opposite force acting back on it”. In this
particular interaction, the push made by the pen
against the wall for example, is the action, while the
action push made by the wall to the pen is the
reaction.
Free-body diagrams however, are diagrams
used to show the relative magnitude and direction of
all forces acting upon an object in a given situation.
The size of the arrow in a free-body diagram reflects
the magnitude of the force. The direction of the arrow
shows the direction that the force is acting. Each force
arrow in the diagram is labelled to indicate the exact
type of force. It is generally customary in a free-body
diagram to represent the object by a box and to draw
the force arrow from the center of the box outward in
the direction that the force is acting.
ANSWER KEY
DEXTER D. PAIRA is a graduate of Bachelor of Secondary
Education Major in Physical Science in the year 2013-2014 at
Negros Oriental State University - Bayawan Sta. Catalina Campus.
He started working at Department of Education in the year 2017.
RONALD G. TOLENTINO is currently teaching in Negros
Oriental High School - Senior High School as STEM 11 Adviser. He
finished Bachelor of Science in Mathematics from Silliman
University and earned 27 units in Master of Arts in Teaching Mathematics at the same University. He finished his CPE from
Presbyterian Theological College.
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