Objectives (5 - 7 minutes)

1. Objective 1: Students will be able to understand and explain the concept of contact forces in Physics, including the forces of friction, tension, and normal forces.

2. Objective 2: Students will learn to identify the effects of contact forces in real-world scenarios, such as the movement of objects on different surfaces, the behavior of ropes, and the dynamics of objects on inclined planes.

3. Objective 3: Through hands-on activities, students will develop the skills to apply the laws and principles of contact forces to solve problems and make predictions about the behavior of objects in various situations.

   Secondary Objectives:

   • Encourage collaborative learning and problem-solving skills through group activities.
   • Foster a positive attitude towards Physics by relating the concepts to real-world applications.
   • Enhance critical thinking skills by challenging students to explain the observed phenomena using the principles of Physics.

Introduction (10 - 12 minutes)

1. Review of previous knowledge (3-4 minutes): To prepare the students for the new material, the teacher will briefly review the basics of forces in Physics, focusing on the concepts of force, mass, and acceleration. The teacher will also remind the students of the difference between contact and non-contact forces, with a quick refresher on gravity as a non-contact force.

2. Problem situations (4-5 minutes): To engage the students and stimulate their thinking, the teacher will present two problem situations related to contact forces:

   • Problem 1: "Why does a box on a ramp move downwards when left alone, but stops when a person pushes it up?"
   • Problem 2: "How does a climber stay on a mountain wall without falling? What forces are involved?"

3. Contextualization and real-world applications (2-3 minutes): The teacher will then contextualize the importance of understanding contact forces by relating it to real-world applications. They will explain that a good understanding of friction, tension, and normal forces can help in various situations, such as designing brakes for a car, understanding how elevators work, or even in sports like rock climbing.

4. Introduction of the topic (1-2 minutes): The teacher will introduce the topic of "Contact Principal Forces" by sharing a fascinating fact or story related to the subject. For instance, they might share the story of how Sir Isaac Newton first observed the force of friction when he tried to push a book across a table. They could also share a curiosity like, "Did you know that without friction, we wouldn't be able to walk or even hold a pencil?"

5. Attention to the students (1 minute): The teacher will grab the students' attention by showing a short video or conducting a quick demonstration related to contact forces. For example, they could show a video of a car drifting on a track, explaining how friction is used to control the car's movement. Or they could demonstrate the force of tension using a rubber band and a small object, showing how the rubber band pulls the object towards it.

Development (20 - 25 minutes)

Activity 1: Friction Investigation (10 - 12 minutes)

1. Introduction of the activity (2 minutes): The teacher will introduce the first activity, where students will investigate the force of friction. They will explain that friction is a contact force that resists the relative motion of two surfaces in contact.

2. Materials needed:

   • A wooden block with a smooth surface.
   • Sandpaper.
   • A ramp inclined at a slight angle.
   • A force meter or spring scale.
   • A data table for recording results.

3. Step-by-step guide:

   • Step 1: Students will each receive the materials needed for the experiment. The teacher will divide the class into small groups to facilitate collaboration.
   • Step 2: Each group will place the wooden block on the ramp and slowly increase the incline until the block starts to move. They will measure the angle of the ramp when the block begins to move and record this value in their data table.
   • Step 3: The groups will then attach sandpaper to the block's surface and repeat step 2.
   • Step 4: Comparing the angles from steps 2 and 3, the students will discuss within their group why the angle is different and what this implies about the force of friction.
   • Step 5: To further explore friction, they will use the force meter to pull the block along the table with and without sandpaper, recording the force required in each case.

4. Discussion of results (5 minutes): The teacher will facilitate a class discussion based on the students' experiment results. They will ask the students to share their findings and observations, guiding the conversation towards the concept that friction is greater between rougher surfaces, as demonstrated by their ramp experiment and the force meter activity.

Activity 2: Tension in Action (10 - 12 minutes)

1. Introduction of the activity (2 minutes): The teacher will introduce the second activity, where students will explore the force of tension. They will explain that tension is a contact force transmitted through a string, rope, or similar object.

2. Materials needed:

   • A rubber band.
   • A small object like a toy car or a light book.
   • A hook or a doorknob.
   • A ruler or a tape measure.
   • A force meter or spring scale.
   • A data table for recording results.

3. Step-by-step guide:

   • Step 1: Students will receive the materials for the experiment and form their groups. They will tie the rubber band between two stable points, making sure it is tight.
   • Step 2: Next, they will carefully hang the small object from the middle of the rubber band. They will measure the length of the rubber band from the hanging object to the rubber band's starting point and record this in their data table.
   • Step 3: Using the force meter, they will apply a force to the rubber band, pulling it to increase its length. They will record the force needed to increase the length by one unit.
   • Step 4: The students will then calculate the amount of tension in the rubber band using the force and length measurements.

4. Discussion of results (5 minutes): The teacher will lead a class discussion based on the students' findings. They will ask the students to share their observations and what they learned about the force of tension. This discussion should highlight that tension increases as the force applied or the length of the object the tension is acting on increases.

The teacher will conclude the development section of the lesson by summarizing the key points from the activities and relating them back to the theory of contact forces, friction, and tension. They will also introduce the last section of the lesson, where students will explore the force of normal force through a short task.