FORCES

SECTION 6: FORCE VIGOUR BEHIND LIFE

Introduction

• Definition of Force: A push or pull that can change the state of motion or shape of an object.
  • Origin of Concept: Recalled from Junior High School science lessons.
• Key Focus Areas:
  • Identify and explain concepts associated with forces, such as:
  • Velocity
  • Acceleration
  • Speed
• Goals: By the end of this section, students should be able to identify and explain concepts associated with forces.

Key Idea

• Force is:
  • A vector quantity, meaning it has both magnitude (size) and direction.
  • Measured in Newtons (N).
  • Fundamental in physics, as it describes interactions between objects and effects changes in motion or shape.

Identification and Explanation of Concepts Associated with Forces

• Role of Forces in Everyday Life:
  • Examples include:
  • Kicking a soccer ball
  • Running
  • Weeding a school farm
  • Other situational examples which might not be immediately identifiable.

Frictional Forces

1. Definition: Forces that arise when two objects physically touch each other.
2. Function: Friction resists relative motion between two surfaces in contact.
3. Benefits of Friction:
   • Prevents slipping; e.g., shoes grip the ground, reducing fall risk.
4. Negatives of Friction:
   • Can hinder motion in moving vehicles; e.g., when trying to accelerate a car.

Activities

Activity 6.1: What are frictional forces?

1. Experiment: Dragging feet on different surfaces (cement/tiled floor vs. bare floor). Share observations with a partner.
2. Discussion Questions:
   • Why do vehicle tyres have textures and markings? Link observations.
3. Figure Analysis (Figure 6.1):
   • A vs. B – Gliding on different surfaces.
   • Questions:
     a. Which moves faster?
     b. Which moves slower?
     c. Which can stop and change direction easily?
   • Justification: Cite reasons from the image to support answers.

Activity 6.2: Exploring the Effects of Frictional Forces

• Aim: Investigate how friction affects sliding distance on surfaces.
• Materials Needed:
  • Wooden block/book/toy car
  • Smooth surface (glass, plastic tray)
  • Rough surface (sandpaper, carpet)
  • Ruler/measuring tape
  • Weighing scale (optional)
• Procedure:
  1. Place a smooth surface on a flat, stable table.
  2. Put the block on the smooth surface and gently push with a constant force, measuring the travel distance before stopping.
  3. Record the distance.
  4. Repeat with the rough surface, maintaining the same pushing force.
  5. Analysis: Discuss challenges of constant force and suggest improvements.
  6. Conclusion: Compare distances covered on both surfaces (smooth vs. rough).

Activity 6.3: Advantages and Disadvantages of Friction

• Discussion: List out benefits and detriments of friction in everyday life.

Gravitational Force

• Definition: The force of attraction between any two objects with mass.
• Function: Responsible for:
  • Keeping planets in orbit around stars.
  • Anchoring objects to Earth's surface.

Activity 6.4: Investigating Gravitational Force

• Aim: Understand gravitational force and its relationship with mass.
• Materials Needed:
  • Two objects of different masses (e.g., small ball and heavier book)
  • Spring scale (Newton meter)
• Procedure:
  1. Calibrate the spring scale before use.
  2. Measure mass of lighter object and record.
  3. Repeat for the heavier object.
• Formula for Gravitational Force:$F = mg$
  • Where:
  • $F$ = gravitational force (N)
  • $m$ = mass (kg)
  • $g$ = acceleration due to gravity ($9.8 \,m/s^2$)

1. Drop both objects from 1 meter height; note the impacting time.
2. Discuss findings with peers.

Velocity

• Definition: Velocity is a vector quantity that indicates the rate of change of an object’s position over time.
  • It has magnitude (speed) and direction.
• Speed Definition: A scalar quantity representing the rate of change of distance with respect to time.
• Word Equation for Velocity:$ext{Velocity} = \frac{ ext{displacement}}{ ext{time taken}}$
  • Displacement is the distance traveled in a given direction.
• Units: Both velocity and speed measured in:
  • Metres per second (m/s)
  • Kilometres per hour (km/h)
• Positive velocity indicates forward motion; negative indicates backward.

Activity 6.5: Exploring Velocity

• Aim: Investigate how velocity relates to everyday scenarios.
• Materials Needed: Stopwatch, measuring tape, toy cars, markers, chart paper, plank/ramp.
• Procedure:
  1. Mark a one-meter length on the ramp.
  2. Lift one end of the ramp and release the toy car to measure time to travel one metre.
  3. Calculate the toy car’s velocity.
  4. Repeat with a higher ramp inclination.
  5. Record results.
  6. Discuss the effects of distance and time on velocity.

Discussion Questions from Figures 6.4 and 6.5

1. Identify distances covered in the races depicted (100m vs. 3,000m).
2. Discuss which race requires a shorter versus a longer time.
3. Discuss potential for greater accelerations in the races.

Distance

• Definition: Scalar quantity representing the total path length covered during motion.
  • Measured in units such as metres (m), kilometres (km), miles (mi).
  • Always positive or zero.

Speed

• Definition: Scalar quantity indicating how fast an object moves.
• Formula for Speed:
  $ext{Speed} = \frac{ ext{distance travelled}}{ ext{time taken}}$
• Units:
  • Metres per second (m/s)
  • Kilometres per hour (km/h)
  • Miles per hour (mph)

Acceleration

• Definition: The rate of change of velocity of an object over time.
  • When an object accelerates, its velocity changes.
• Acceleration Formula:$a = \frac{(v - u)}{t}$
  • Where:
  • $a$ = acceleration (m/s²)
  • $u$ = initial velocity
  • $v$ = final velocity
  • $t$ = time (s)
  • Negative acceleration indicates deceleration.
  • Uniform acceleration: Constant acceleration.

Resultant Force and Acceleration

• Newton’s Laws:

1. First Law: An object’s velocity remains unchanged unless acted on by a resultant (net) force.
2. Second Law:
   • Resultant force is related to acceleration:
     $F = ma$
   • Where:
     • $F$ = force (N)
     • $m$ = mass (kg)
     • $a$ = acceleration (m/s²)

Activity 6.6: Examples

1. Runner Distance: 200 meters in 20 seconds → Average Velocity:
   $ext{Average Velocity} = \frac{200}{20} = 10 \, ext{m/s}$
2. Car Distance: 300 km in 5 hours → Average Speed:
   $ext{Average Speed} = \frac{300}{5} = 60 \, ext{km/h}$
3. Car Mass: 1500 kg accelerates from 0 to 20 m/s in 10 s → Net Force:
   $F = ma = 1500 imes 2 = 3000 \, ext{N}$
4. Car Speed: 200 km in 2 hours → Speed:
   $ext{Speed} = \frac{200}{2} = 100 \, ext{km/h}$
5. Car Acceleration: from 0 m/s to 20 m/s in 5 seconds:
   $ext{Acceleration} = \frac{20 - 0}{5} = 4 \, ext{m/s}²$

Activity 7: Experiment about Acceleration (Gravity)

• Materials Needed: Toy car or small rolling object, smooth flat surface, measuring tape or ruler, stopwatch, notebook.
• Procedure:
  1. Set up a surface with enough room for acceleration.
  2. Mark uniform distances along a surface.
  3. Start the stopwatch when the object is released.
  4. Measure time to reach each mark.
  5. Calculate average speed with:
     $ext{Speed} = \frac{ ext{Distance}}{ ext{Time}}$
  6. Analyze speed to determine if the object is accelerating or decelerating.
  7. Calculate acceleration:
     $ext{Acceleration} = \frac{ ext{Final Velocity - Initial Velocity}}{ ext{Time}}$
  8. Document observations, measurements, and conclusions.

Cohesive and Adhesive Forces

• Cohesive Forces: Attractive forces within molecules of the same substance (e.g., water molecules).
• Adhesive Forces: Attractions between different substances (e.g., water adhering to glass).

Activity 6.8: Understanding Cohesive and Adhesive Forces

• Objective: Differentiate between cohesive and adhesive forces through experiments.

Experiment 1: Cohesive Forces

• Materials Needed: Containers, water, salt, small objects, droppers.
  • Stir salt into water and compare droplet behaviour.
  • Observe how saltwater droplets differ from plain water droplets.

Experiment 2: Adhesive Forces

• Materials Needed: Capillary tubes, tissue, water, food coloring.
  • Observe how water rises in capillary tubes.
  • Measure water column height in different tubes.

Activity 6.9: Real-life Applications of Force

Activity 10: Summary Creation

• Produce a summary sheet or poster on forces and motion. Suggest including:
  • Glossary of key terms.
  • Differences between:
  • Distance vs. displacement
  • Speed vs. velocity
  • Acceleration
  • Examples from everyday life or personal interests.

Annex 6.1 – Solutions to Activities

1. Activity 6.2: Shorter distance on rough surface due to increased friction.
2. Activity 6.4: All masses fall at the same rate; gravitational force is consistent.
3. Activity 6.5: Greater height results in less time for descent, leading to increased velocity.
4. Activity 6.6: Use results from velocity and speed calculations.
5. Activity 6.8: Observations about adhesion and cohesion and their role in capillarity.

Review Questions

1. Soccer Match Force Example:
   a. Acceleration after kick: $a = \frac{F}{m} = \frac{50}{0.5} = 100 \, ext{m/s}²$
   b. Consider external forces like air resistance, friction on the ground, and drag.
2. Stuck Car: Factors preventing movement can include friction and surface condition. Strategies to get moving: leverage, pushing, or using other vehicles.
3. Gravitational Influence: Discuss how gravity maintains orbit and affects motion in the Solar System.
4. Interactions with Fluids: Consider the effects of cohesive and adhesive forces on motion and fluid dynamics.

References

1. General Science Curriculum for Senior High Schools
2. Smith, J., & Johnson, M. (2021). Teaching Forces: Strategies for Engaging Students in Physics Concepts. Journal of Science Education, 15(2), 45-56.
3. Brown, A., & Williams, R. (2019). Interactive Approaches to Teaching Forces in Middle School Science. Journal of STEM Education, 8(3), 112-125. Oxford University Press. Complete Physics for Cambridge IGCSE, Third Edition.
4. Blanchet, L. (2006). Gravitational radiation from post-Newtonian sources and inspiralling compact binaries. Living Rev. Relativity 9.
5. Blanchet, L., and Faye, G. (2000). Hadamard regularization. Math. Phys. 41, 7675–7714.
6. Kennefick, D. 2005. Einstein versus the Physical Review. Physics Today 58(9), 43–48.
7. Kennefick, D. 2007. Traveling at the Speed of Thought: Einstein and the Quest for Gravitational Waves. Princeton University Press.

Glossary

• Celestial Body: Any natural object in space with physical substance, distinguishing them from phenomena such as light or radiation.

Acknowledgements

• Contributors:
  • Prof. Christian A. Krueger, UCC, Cape Coast
  • Emmanuel O. Ocquaye, GES, Science Education Unit, Accra
  • Rev. Thomas K. Arboh, Police Education Directorate, Accra
  • Samuel Bismark Larbi, Mfantsipim School, Cape Coast