FORCES AND MOTION
LEARNING COMPETENCIES:






The Learners should be able to…
1. investigate the relationship between the amount of force applied
and the mass of the object to the amount of change in the object’s
motion; S8FE-Ia-15
2. infer that when a body exerts a force on another, an equal
amount of force is exerted back on it; S8FE-Ia-16
3. demonstrate how a body responds to changes in motion;
S8FE-Ib-17
4. relate the laws of motion to bodies in uniform circular motion;
S8FE-Ib-18
5. infer that circular motion requires the application of constant
force directed toward the center of the circle; S8FE-Ib-19
FORCES AND MOTION
BALANCED AND
UNBALANCED FORCES
Objectives
a. Identify the forces acting on an object
b. Describe the forces acting on an object
FORCE
A PUSH OR PULL ON A BODY IS CALLED FORCE.
FORCES ARE USED IN OUR EVERYDAY ACTIONS LIKE
PUSHING, PULLING, LIFTING, STRECHING, TWISTING
AND PRESSING.
EFFECTS OF FORCE
A force cannot be seen. A force can be judged only by the
effects which it can produce in various bodies (or objects)
around us.
1. A force can move a stationary body.
2. A force can stop a moving body.
3. A force can change the speed of a moving body.
4. A force can change the direction of a moving body.
5. A force can change the shape and size of a body.
•
Questions:
1.What makes objects move the way they
do?
2.Why do objects move in different ways?
Why are some objects faster than the
others?
3.What makes objects stay in place?
-
Activity 1 Forces on objects at rest
Q1. Is the pen at rest or in motion?
Ans. The pen is at rest
Q2.Are the forces acting on the pen? If yes, draw
the forces.You may use arrows to represent
these forces.
Ans. The forces acting on the pen are tension
force(force exerted by the string on the pen) and
force of gravity
Q3 What happens to the pen? What could
have caused the pen’s motion?
Ans. When the string was cut, the pen falls
to the ground.The force of gravity makes the
object fall down.
Q4. Is the book at rest or in motion?
Ans. The book is at rest
Q5 Are there forces acting on the book? If yes,
draw the forces acting on the book.
Ans. Yes The forces acting on the book are the
force exerted by the table on the book(Normal
Force) and force of gravity
Q6 Did the book move? How will you make the
book move?
Ans. No.The book is at rest.The book maybe
moved by pushing it on one side only.
ACTIVITY 2
RECOGNIZING FORCES
ACTIVITY 3
CHANGES BROUGHT ABOUT BY FORCES
ACTIVITY 4
MAKING AN OBJECT MOVE
SELF TEST
ACTIVITY 5
DIFFERENT TYPES OF FORCES
Key Concepts
FORCE-push or a pull that can make an
object moves
.
-can make objects move,move
faster,stop or change their direction of
motion.
-change the shape of things
-are described in terms of the
properties such asmagnitude,
direction,point of application
and line
of action.
Magnitude-refers to the size or strength of the force
and commonly expressed in Newton.( 5N,3N)
Direction of the arrow-direction of the force.
Length of the arrow-represents the relative
magnitude of the force.
If the force of an object increased,the length of the
arrow also increases.
Line of Action-is the straight line passing through the
point of application.
Point of Application-is parallel to the direction of the
force.
• If an object stays at rest, there are forces
acting upon it.
• Hanging objects have tension force and
force of gravity acting on them
• Objects at rest have normal force and
force of gravity acting on them
• Tension Force-Force exerted by the string
on the object
• Normal Force is the upward force
perpendicular to the surface of contact
between two bodies
OBJECTIVES
a.Differentiate balanced from
unbalanced force
b. Calculate the resultant force
Balanced vs. unbalanced forces
• Unbalanced: when the net force on
an object is not zero. These
produce a change in motion.
• Balanced: when the net force on an
object equals zero. These do NOT
produce change in motion.
Balanced Force-the forces acting on an
object are equal in magnitude but
opposite in direction.
-lie along the same line of
action.
-the object either stays at
rest or
continues to move at constant velocity.
Friction-acts opposite the
direction of motion.
-slows down/resists the
motion of an object.
Due to unbalanced force, the object
change its state of motion hence we say
that it accelerates or increase in velocity
but it can also decelerate or decrease in
velocity.
Unbalanced Force-the motion of the
object will change.
ACTIVITY 1
BALANCED VS. UNBALANCED FORCES
Some tips:
1. Forces in the same direction- add the two
forces together.
+
=
1. Forces in different directions- subtract
the two and figure out which direction was
the stronger of the two.
=
To determine the net force,use the algebraic
signs + and – to indicate the directions of
forces.
Fnet=F1 + F2 +……(if the forces of an object
are in the same direction)
Fnet= F1-F2-…….(if the
forces of an
object are in
opposite direction)
LAWS OF MOTION
OBJECTIVES:
a. State
and Explain Newton’s three
laws on motion
b. Explain the concept of inertia and
give examples
c. Apply the Laws of Motion to simple
situations
d. Solve problems using the second
law of motion
e. Differentiate forces related to Law
of Interaction and Forces in a
Balanced state
NEWTON’S LAW OF MOTION
•
NEWTON HAS GIVEN THREE LAWS
OF TO DEFINE THE MOTION OF
BODIES. THESE LAWS ARE KNOWN
AS NEWTON’S LAWS OF MOTION.
Sir Isaac Newton-one of the
greatest scientists and
mathematicians, he formulated
the laws regarding motion and
forces.
1.Law of Inertia
2.Law of Acceleration
3. Law of Interaction
NEWTON’S FIRST LAW OF MOTION
-an object at rest will
remain at rest or an
object in motion will
stay in motion and
travel in straight line
unless acted upon by an
external net or
unbalanced force.
Newton’s First Law (law of inertia)
• Mass (kg)is the measure of the
amount of matter in an object.
• INERTIA is a property of an object
that describes how much it will
resist change to the motion of the
object
Inertia-a property of an object to
resist any change in its state of
motion.
More massive object which has more
inertia is more difficult to move from
rest,slow down,speed up or change
its direction.
ACTIVITY 1
I CAN’T STOP MOVING!
SELF TEST
ACTIVITY 2
INERTIA AND MASS
Law of Acceleration/ Second Law of Motion
-acceleration of an object
is directly proportional to
force and inversely
proportional to its mass.
This will help up remember:
Force is expressed in Newton (N).
2
1N=1kg.m/s .
The SI Unit for acceleration if
2
m/s (meter per second square),
for mass is kg(kilogram) and for
force is Newton(N).
If the acceleration of an
object increases, the force of
it also increases.
If the acceleration of an
object decreases, the force of
it also decreases.
If the acceleration of an
object increases, the mass of
it decreases.
If the acceleration of an
object decreases, the mass of
it increases
.
ACTIVITY 1
NEWTON’S SECOND LAW PROBLEM
SOLVING
Freefall and Second Law of Motion.
Galileo-found out that two bodies
dropped at the same height will reach
the ground at the same time regardless
of their masses.
(without air resistance).
Aristotle-he proved the observation of
Galileo in the Leaning Tower of Pisa.
Gravity-acts on all objects on the
earth’s surface and causes them to
accelerate when released.This is known
as the acceleration due to gravity
2
and it is equal to 9.8m/s .
Freefall and Second Law of Motion.
Galileo-found out that two bodies dropped at the same height
at the same height will reach the ground
at the same time regardless of their masses.(without
air resistance).
Aristotle-he proved the observation of Galileo in the Leaning
Tower of Pisa.
Gravity-acts on all objects on the earth’s surface and causes
them to accelerate when released.This is known as the
acceleration due to gravity and it is equal to 9.8m/s2.
Circular Motion and Second Law of Motion
Circular Motion-motion of an object along a circle.
2 Forces in a Circular Motion
1.Centripetal Force-the inward force the pulls the
object towards the center of the circle.
2.Centrifugal Force-the outward force the pulls the
object away the center of the circle.
NEWTON’S THIRD LAW OF MOTION
Whenever one body exerts a force on another body,
the second body exerts equal and opposite force to
the first body. OR,
To every action there is equal and opposite reaction.
1. EX: the rocket works on the principal of action
and reaction.
2. When a bullet is fired from the gun, a force
sending the bullet forward is equal to the force
sending the gun backward. But due to high mass
of the gun, it moves only a little distance
backward and gives jerk.
Newton’s 3rd Law
• For every action there is an equal and opposite
reaction.
Book to
earth
Table to
book
Think about it . . .
What happens if you are standing on a skateboard or
a slippery floor and push against a wall? You slide in
the opposite direction (away from the wall), because
you pushed on the wall but the wall pushed back on
you with equal and opposite force.
Why does it hurt so much when you stub your toe?
When your toe exerts a force on a rock, the rock
exerts an equal force back on your toe. The harder
you hit your toe against it, the more force the rock
exerts back on your toe (and the more your toe
hurts).
Action and Reaction on Different Masses
Consider you and the earth
Action: earth pulls on you
Reaction: you pull on earth
Reaction: road pushes on tire
Action: tire pushes on road
Reaction: gases push on rocket
Action: rocket pushes on gases
This will help up remember:
Newton’s Third Law video clip
ACTIVITY 1
ACTION-REACTION FORCES
ACTIVITY 2
NEWTON’S THIRD LAW
Differences between forces related to
Law of Interaction and Forces in a Balanced state:
Action-Reaction Forces
 Two forces are equal in size
 Two forces are opposite to each
other in terms of direction.
 Two forces have the same line of
action
 Action acts on one object,while
reaction acts on another object.
Balanced Forces
 Two forces are equal in size
 Two forces are opposite to
each other in terms of
direction.
 Two forces act along the
same line of action
 Two forces act upon the
same object