FORCE AND ENERGY Notes, Quizzes & Revision
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Subtopic: FORCE AND ENERGY
Target learners: age_replace (Kenyan context). These notes follow concepts commonly taught in Kenyan primary/secondary science and applied physics at the appropriate level for the age_replace learner.
- Define and identify force and its effects (start, stop, change direction or shape of objects).
- Distinguish between contact and non-contact forces (e.g., push/pull vs gravity, magnetism).
- Understand and use simple equations: F = m a, weight W = m g, work W = F d, KE = Ā½ m vĀ², PE = m g h.
- Describe common forms of energy and energy transformations (kinetic, potential, heat, light, chemical, electrical).
- Solve simple calculation problems and carry out safe practical activities demonstrating forces and energy in Kenyan contexts (e.g., lifting jerrycans, loading sacks).
1. What is a force?
A force is a push or pull that can cause an object to start moving, stop, change direction, or change shape. Forces are measured in newtons (N).
Contact forces: friction, tension, normal (support) force, applied push or pull.
Non-contact forces: gravity, magnetic, electrostatic.
2. Important force facts and equations
- Newton's second law: F = m a (force in N, mass in kg, acceleration in m/sĀ²).
- Weight (force due to gravity): W = m g (g ā 9.8 m/sĀ²; often approximated as 10 m/sĀ² for simple calculations).
- Resultant force determines motion: if resultant ā 0, object accelerates; if resultant = 0, object at rest or constant speed.
3. Energy ā forms and examples
Energy is the ability to do work. Common forms:
- Kinetic energy (KE) ā energy of motion: KE = Ā½ m vĀ² (J).
- Gravitational potential energy (PE) ā due to height: PE = m g h (J).
- Chemical energy ā stored in food or fuels (e.g., wood, petrol).
- Heat (thermal) energy ā from burning, friction, or heating.
- Light and electrical energy ā from lamps, batteries, solar cells.
Conservation of energy: energy cannot be created or destroyed; it changes form (e.g., chemical ā kinetic ā thermal).
4. Worked examples (Kenyan contexts)
A sack of maize has mass 25 kg. Find its weight. Take g = 9.8 m/sĀ².
Solution: W = m g = 25 Ć 9.8 = 245 N.
A water jerrycan of mass 10 kg is lifted 1.5 m to a shelf. Work done against gravity? (Use g = 10 m/sĀ² for easy estimate)
Solution: W = m g h = 10 Ć 10 Ć 1.5 = 150 J.
A biker and motorbike (total mass 120 kg) travel at 10 m/s. Find KE.
Solution: KE = Ā½ m vĀ² = 0.5 Ć 120 Ć (10)Ā² = 0.5 Ć 120 Ć 100 = 6000 J.
5. Simple investigations and classroom activities
- Measure how effort (force) needed increases as you add weight to a box using a spring balance; discuss friction and normal force.
- Drop objects of different mass from the same height to show that gravitational acceleration is similar (time measured with stopwatch) ā safe, small heights only.
- Lift a known mass to different heights and calculate PE = m g h; check that lifting higher requires more work.
- Use a simple ramp to show how force along the ramp changes compared to lifting directly (compare effort and distance).
Safety note: Always work in small groups, avoid heavy lifting without assistance, wear closed shoes, and follow teacher instructions.
6. Practice questions (try these)
- A box of mass 15 kg is pushed across a floor with a horizontal force of 30 N for 4 m. How much work is done by the force? (Answer: W = F d = 30 Ć 4 = 120 J)
- A child of mass 30 kg climbs a 2 m high tree stump. Calculate the increase in gravitational potential energy (use g = 9.8 m/sĀ²).
- A car of mass 800 kg speeds up from 0 to 20 m/s. Find its kinetic energy.
- Give two examples of energy transformation in a Kenyan household.
Key terms (quick reference)
Force (N), Mass (kg), Acceleration (m/sĀ²), Weight (N), Work (J), Energy (J), Power (W), Kinetic energy, Potential energy, Conservation of energy.