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 objectb. Describe the forces acting on an object

FORCEA 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 2RECOGNIZING FORCES

ACTIVITY 3CHANGES BROUGHT ABOUT BY FORCES

ACTIVITY 4MAKING AN OBJECT MOVE

SELF TESTACTIVITY 5

DIFFERENT TYPES OF FORCES

Key ConceptsFORCE-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 forceb. 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 1BALANCED VS. UNBALANCED FORCES

Some tips:1. Forces in the same direction- add

the two forces together. + =

2. 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 motionb.Explain the concept of inertia and give examplesc.Apply the Laws of Motion to simple situationsd.Solve problems using the second law of motione.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 Inertia2.Law of Acceleration3. 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 1I CAN’T STOP MOVING!

SELF TESTACTIVITY 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).

1N=1kg.m/s2.

The SI Unit for acceleration if m/s2(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 1NEWTON’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 and it is equal to 9.8m/s2.

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 MotionCircular Motion-motion of an object along a circle.

2 Forces in a Circular Motion1.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 MOTIONWhenever 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 toearth

Table tobook

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: earth pulls on you

Reaction: you pull on earth

Action and Reaction on Different Masses

Consider you and the earth

Action: tire pushes on roadReaction: road pushes on tire

Action: rocket pushes on gases

Reaction: gases push on rocket

This will help up remember:Newton’s Third Law video clip

ACTIVITY 1ACTION-REACTION FORCES

ACTIVITY 2NEWTON’S THIRD LAW

Action-Reaction Forces Balanced 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.

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

Differences between forces related to Law of Interaction and Forces in a Balanced state: