SEMI-DETAILED LESSON PLAN

Science Grade 8

I. OBJECTIVES
1. Infer that when a body exerts a force on another, an equal amount of force is exerted
back on it
2. Explain the phenomenon behind the action and reaction forces
3. Cite real-life situations in which the 3rd law of motion can be depicted

II. SUBJECT MATTER

III. PROCEDURE
A. Preparatory Activity

Let us all stand for a prayer. Kym kindly lead the prayer.
Class secretary please check the attendance.

1. Drill/Review
To make everyone lovely, let us all stand and do some exercise. We must not forget to
live a healthy life.
I will show you a video and all you have to do is to copy the actions of the dance
instructor in the video. Am I clear class?

2. Motivation
Before we start our proper discussion, let us first have a game. Do you love to play games
class?
I see that you are eager to play the game for today class. The name of the game is
“Decode Me”. Now, I will group the class into four(4). This will be team royals, team
beauties, team fabulous and team dreamers. Each team will now form a circle to be able
to communicate with each member. Now class I want you to look at the board. In the
board, you will see numbers and letters which corresponds to the codes that I will provide
you later. Your task is to right the corresponding letters to the codes that I will present to
you on the board.
Congratulations team_____! Let us all give them a Dionisia clap.

Now class, what can you say about the pictures you have solved?

3. Presentation of New Lesson

What do you think is our topic for today?

I will be presenting to you a short video about the background of the motions third law
which is the law of interaction. Remember to take down notes and comprehend the
principle and ideas behind the law for we will be having an activity after the video is
presented. Am I clear class?
(the video is being presented to the class)

To make things exciting, let us have our discussion through an activity. With your group,
you are to create a short play that shows or illustrates the law of interaction within 3 to 5
minutes. At the end of your play, you will state a short explanation about the concept of
your short play to enlighten everyone. You will be judged according to these criteria.
(teacher shows the criteria) The manner of whose team goes first will be decided in a
lottery game. The group to pick the #1 will be the first to present and son and so forth.
Any questions?

You will be given 15 minutes to prepare for the activity. Am I understood?

Very good! Everyone did a wonderful performance.

Class, remember that the third law states that when a body A exerts a force on body B, it
will exert an equal but oppositely directed force on body A. That’s what it meant when
they said “to every action, there is equal but opposite reaction force”. For example, if I
touch the table, of course my hand exerted a force upon the table, does the table also
exerts a force in my hand?

How much of a force did the table exerted?

Very good! That is how the third law of motion works. You cannot touch without being
touched.
If you push hard on a wall with a force of 5N. The wall in turn pushes you back by
a force of 5N. We can walk because we push backward upon on earth as hard as the earth
pushes us forward. Another popular example is the horse cart paradox. Have you heard
about it before class?
Before I tell you the story, I have a question. Based on the given definition and
example of the third law of motion, do action and reaction forces cancel out each other?

Well, let me tell you the story, “One morning, a trainer and his horse were having
an argument. The horse that happens to know a little physics told his trainer that it is
 useless for the former to pull the cart. The harder the horse pulls the cart, the harder the
cart pulls back on him. These forces will cancel each other and the net forces will be zero
in as much as action and reaction forces are supposed to be equal but opposite directed.
The horse further said that since there is no net force, then the cart will remain at rest in
accordance with Newton’s first law. If you are the trainer, will you agree with the horse?

Of the two forces mentioned by the horse, it is only the force that the horse
exerts on the cart that is acting on the cart. The other force in the action-reaction force is
acting on the horse and not on the cart. Since action and reaction forces are acting on
two different objects, they will never cancel out. The cart will accelerate if the forces
exerted by the horse on the cart is greater that the force of the friction between the cart
and the ground.

IV. GENERALIZATION

Now, go back to the question earlier, do action and reaction forces cancel each other?

What does the third law says?

Any questions class?

V. EVALUATION

To test your understanding of the lesson, we will have a short quiz. Get ½ sheet of paper and
answer the questions on the board.

An object of weight 30 N rests a table.

a. What are the forces acting on the object?

VI. ASSIGNMENT

To strengthen more your knowledge of today’s lesson, let us have an assignment.

Write this in a ½ crosswise sheet of paper.

A 650 students stand on a scale in an elevator. As the elevator starts to move, the scale reads
780 N.

1. Find the acceleration.