• What forces are and how we measure them
• Newton's laws in simple terms
• Forms of energy and how energy changes (conservation)
• Work, power and calculations with simple examples
• Local (Kenyan) examples to connect concepts to everyday life

1. Force — definition and units

Force is a push or a pull that can change the motion of an object (make it speed up, slow down, stop or change direction). The SI unit is the newton (N).

Common forces: gravity (weight), normal force, friction, tension, and magnetic force.

2. Newton's Laws (simple)

1. 1st law (Inertia): An object at rest stays at rest, and an object moving keeps moving unless a force acts on it. Example: a ball on a mat stays put until pushed.
2. 2nd law (F = ma): The acceleration of an object is proportional to the net force and inversely proportional to its mass.
   Formula: F = m × a (force in N, mass in kg, acceleration in m/s²)
3. 3rd law (Action–Reaction): For every action there is an equal and opposite reaction. Example: when you jump, your feet push the ground down and the ground pushes you up.

3. Energy — types and key ideas

Energy is the ability to do work. It can be stored (potential) or in motion (kinetic).

• Kinetic energy (KE): energy of motion. KE = 1/2 m v² (m in kg, v in m/s).
• Gravitational potential energy (PE): stored energy due to height. PE = m g h (g ≈ 9.8 m/s²).
• Other forms: thermal, chemical (food, petrol), electrical, nuclear, elastic.

4. Work and Power

Work is done when a force moves an object: Work = Force × distance × cosθ. Unit: joule (J).

Power is rate of doing work: Power = Work / time. Unit: watt (W) where 1 W = 1 J/s.

5. Conservation of Energy

Energy cannot be created or destroyed, only changed from one form to another. In many practical situations some energy becomes heat due to friction.

Example: A cyclist at the top of a slope has gravitational potential energy that becomes kinetic energy as they go down; brakes convert some energy to heat.

6. Kenyan context — everyday examples

• Car brakes on a busy matatu: friction converts kinetic energy to heat, slowing the vehicle.
• Jembe (hoe) used in farms: force and work when digging soil; muscular chemical energy becomes work.
• Hydroelectric dams and Olkaria geothermal plants: conversion of potential (water at height) or geothermal heat into electrical energy.
• Wind turbines at Lake Turkana: wind (kinetic energy of air) converted to electrical energy.

7. Worked problems (step-by-step)

1. Weight from mass: A student of mass 50 kg stands on a scale. Find the weight.
   Weight W = m g = 50 × 9.8 = 490 N (approx).
2. Kinetic energy: A jembe (tool) of mass 0.5 kg is swung so its end moves at 4 m/s. Find KE.
   KE = 1/2 m v² = 0.5 × 0.5 × 4² = 0.25 × 16 = 4 J.
3. Using F = ma: A boda-boda accelerates at 2 m/s² and its rider+bike mass = 100 kg. Find net force.
   F = 100 × 2 = 200 N.

8. Simple classroom activities (safe, low-cost)

• Drop a small ball from different heights and measure time to reach ground. Discuss PE to KE and why times differ.
• Use a spring balance to measure force needed to pull a textbook across desk vs. on carpet (friction effects).
• Roll a toy car down ramps of different heights; measure distance travelled and discuss energy changes and friction losses.

9. Key terms to remember

Force, mass, weight, inertia, acceleration, kinetic energy, potential energy, work, power, conservation of energy, friction.

10. Quick revision questions

1. State Newton's second law in words and give the formula.
2. What is the unit of energy? Give an example of chemical energy used at home.
3. Calculate the work done if a force of 15 N moves a shopping bag 2 m horizontally.
4. Explain, using energy ideas, why brakes heat up when used on a long descent.