Lesson 2 • Balanced and unbalanced forces

Recall the previous lesson, focusing on forces in sport and the definitions of force. Discuss any questions the students may have about forces. Use the Forces and sport Poster as a visual prompt for students if needed.

(Slide 11) Invite students to find examples of these statements about forces in the picture of the bike rider shown on the slide.

• A force is a push or pull that acts on an object when it interacts with another object.
• Some forces cause movement such as changing direction or speeding up or slowing down.
• Other forces are balanced, and the object’s motion remains unchanged.

(Slide 12) Pose the questions: What happens when forces act in different directions? Why?

(Slide 13) Explain that forces have both size (which is called “magnitude”) and a direction.

Referring to the sports images shown on the slide, ask students to identify forces acting in different and similar directions and to identify what the force is acting on.

Explain that forces can act on the same object or different objects. If forces are acting on the same object, they can balance, resulting in no changes in motion. Alternatively, forces can be unbalanced and lead to starting motion, stopping motion, or result in a change of speed or direction. Invite students to identify if the forces acting on an object are causing or stopping movement.

✎STUDENT NOTES: Write a sentence describing forces having a size/magnitude and direction.

Discuss how students could investigate the lesson question using a small model.

(Slide 14) Introduce the model shown on the slide and discuss how elastic bands provide forces acting on the block.

Provide students with a copy of Investigating forces - Forces in different directions Resource sheet.

Students read through the investigation, allocate roles, collect equipment and conduct the investigation.

Forces in different directions investigation—procedure

1. Place the wooden block and pin on the butchers paper. Put two elastic bands around the pin/nail. Trace around the block to show your starting position.
2. One team member holds the block stationary inside the traced shape. Two other team members gently pull the elastic bands equal distance (or force) from either end of the block.
3. Another team member draws accurate arrows on the butchers paper alongside each stretched band, showing the distance they are stretched. Label these arrows “Pull force left” and “Pull force right”.
4. Release the bands at the same time. Draw an accurate arrow to indicate the distance the edge of the block travelled in the direction of either band. Label the arrow as “Final force”, measure and record in the results table. Repeat the experiment another two times if time allows.
5. Repeat steps 1 and 2, but this time the two team members pull back the bands different distances: one pulls their band same distance as previously, the other pulls their band twice the distance as previously.

6. Another team member draws accurate arrows on the butchers paper alongside each stretched band. Label these arrows “Pull force left” and “Pull force right”.
7. Release the bands at the same time. Draw an accurate arrow to indicate the distance the edge of the block travelled in the direction of either band. Label the arrow as “Final force”, measure and record in the results table. Repeat the experiment another two times if time allows.

✎STUDENT NOTES: Complete the Results table of Investigating forces—Forces in different directions Resource sheet.

If required, guide students through the process of calculating an average of the three trials using this formula:

$$ \text{Average}=\frac{\text{distance}_\text{trial 1}+\text{distance}_\text{trial 2}+\text{distance}_\text{trial 3}}{\text{number of trials}}$$

Optional: If time allows, extend students’ understanding of unbalanced forces by completing the Forces in the same direction investigation.

Optional investigation: Forces in the same direction

(Slide 15) Pose the questions: What happens when forces act in the same direction? Why?

Provide students with a copy of Investigating forces - Forces in the same direction Resource sheet.

Students read through the investigation, allocate roles, collect equipment and conduct the investigation.

Procedure

1. Place the wooden block and pin on the butchers paper. Put an elastic band around the pin/nail. Trace around the block to show your starting position.
2. One team member holds the block stationary inside the traced shape. One team member pulls one elastic band the same distance (or force) as the previous experiment on one side of the block.
3. Another team member draws an accurate arrow on the butchers paper alongside the stretched band. Label this arrow as “Pull force 1”.
4. Release the band. Draw an accurate arrow to indicate the distance the edge of the block travelled in the direction of the band. Label the arrow as “Final force”, measure and record in the results table. Repeat the experiment another two times if time allows.

5. Place the block with pin on a new piece of butchers paper. Put an elastic band around the pin/nail. Trace around the block to show your starting position.
6. One team member holds the block stationary inside the traced shape. One team member pulls two elastic bands the same distance (or force) as the previous experiment on one side of the block.
7. Another team member draws accurate arrows on the butchers paper alongside each of the stretched bands. Label these arrows as “Pull force 1” and “Pull force 2”.
8. Release the bands at the same time. Draw an accurate arrow to indicate the distance the block travelled in the direction of the bands. Label this arrow as “Final force”, measure and record in the results table. Repeat the experiment another two times if time allows.

✎STUDENT NOTES: Record the distance travelled by the wooden block in the results table.

Discuss and compare students’ results for the investigation/s. Invite students to explain why the block moved in one instance and not in the other.

(Slide 17) Explain that in the first part of the investigation, the forces were balanced (of equal magnitude or size and acting in opposite directions on the block). This resulted in a zero net force and the block remains stationary.

(Slide 18) Explain that in the second part of the investigation, the forces were acting in opposite directions on the block, but they were not of equal size or magnitude, so the net force was not zero. These unbalanced forces caused motion to occur. The net force can be calculated by subtracting the forces to determine the direction of motion.

✎STUDENT NOTES: Complete the Discussion section of Investigating forces - Forces in different directions Resource sheet.

If you completed the Forces in the same direction investigation

Discuss and compare the results of the two force scenarios in the optional investigation. Ask students to explain the movement of each block using force arrows.

(Slide 19) Guide students through the force diagram to show the unbalanced force on the block and the net force in each scenario. Draw the force diagrams to aid your explanation.

One band:

Two bands in the same direction: 

Explain that when forces act on the object in the same direction, the net force is determined by adding the forces.

✎STUDENT NOTES: Complete the Discussion section of the Investigating forces—Forces in the same direction Resource sheet.

(Slide 20) Introduce how scientists use different models to help them understand, explain, or test their hypotheses. Discuss how the investigation used a physical model to test how different forces interact to produce a net force.

Explain that all models are limited, and this physical model is limited to small forces in only a few directions because that is what can be produced in the classroom. Introduce the PhET Forces and motion: basics ‬computer simulation model. This model allows students to explore their ideas through real-world examples without needing to physically do the activity.

Distribute the Net force simulation Resource sheet. Explain that in this activity, students will be using their understanding of net force to complete two challenges: creating a situation where forces on an object are balanced and creating a situation where the forces are unbalanced.

Allow students time to work through the instructions for using the simulation and the two challenges, given on Net force simulation Resource sheet.

Two challenges

1. Select the Net force simulation.

2. Challenge 1: Challenge students to create a scenario using at least one person in which the forces are balanced and the cart is not moving. Point out to students that the net or final force in this simulation is called the “Sum of forces”.

3. Challenge 2: Challenge students to create a scenario using at least two people in which the forces are unbalanced.

✎STUDENT NOTES: Record the results of the challenges, draw a force diagram showing force arrows, and calculate the net force on the Net force simulation Resource sheet.

(Slide 21) Guide students in identifying the balanced and unbalanced forces on the barbell in the three pictures. Relate what students have learnt about balanced, unbalanced and net forces to the pictures.

(Slide 22) In small groups, students work together to:

• choose a sport or physical activity.
• identify situations where forces are balanced and where forces are unbalanced.
• explain how these forces affect motion.

If students find choosing an activity or sport difficult, provide the following examples: tennis, golf, table tennis, air hockey or tug-of-war.

✎STUDENT NOTES: Write an explanation of how motion is affected in two situations where forces acting on an object are balanced and unbalanced in a chosen sport or physical activity.

Reflect on the lesson

You might invite students to:

• review the learning goals for the lesson and check for understanding.
• write a list of the important forces of a sport or physical activity to show in an infographic.
• identify five examples in their sport or physical activity where forces are acting in opposite directions on an object (like a person or piece of equipment). For each example, they need to record:
  • what the object is.
  • the two opposing forces.
  • whether the forces are balanced or unbalanced.
  • the evidence (e.g. “the bin isn’t moving → forces are balanced”).
  • a force diagram for their favourite example.
• write a paragraph explaining why forces are important to playing or participating in their chosen sport or physical activity.
• add the words about forces covered in this lesson to their glossary.