FORCE AND ENERGY Notes, Quizzes & Revision
๐ Revision Notes โข ๐ Quizzes โข ๐ Past Papers available in app
topic_name_replace โ FORCE AND ENERGY
Subject: subject_replace | Target age: age_replace | Fit to Kenyan context
Specific learning outcomes
- Define force and energy and state their SI units (newton, joule).
- Differentiate types of forces (contact and non-contact; balanced and unbalanced).
- Explain work, power and simple energy transformations (kinetic, potential, thermal, light, electrical).
- Apply formulas W = F ร d and P = W / t to solve simple numerical problems.
- Describe the law of conservation of energy and give everyday Kenyan examples.
Key terms (quick)
Force, mass, weight, resultant force, balanced force, unbalanced force, newton (N), energy, work, joule (J), kinetic energy (KE), potential energy (PE), power (P), watt (W), conservation of energy.
1. Force โ idea and types
A force is a push or a pull that can change the motion of an object. It is a vector (has size and direction). SI unit: newton (N).
Types
- Contact forces: applied force, friction, normal reaction, tension.
- Non-contact forces: gravity (weight), magnetic force, electrostatic force.
- Balanced forces: equal forces in opposite directions โ no change in motion.
- Unbalanced forces: net force โ 0 โ object accelerates.
2. Weight vs mass
Mass is the amount of matter (kg). Weight is the gravitational force on the mass. Weight = mg where g โ 9.8 m/sยฒ (โ10 m/sยฒ for quick estimates).
Example: A mass of 5 kg has weight โ 5 ร 9.8 = 49 N.
3. Energy โ concepts and forms
Energy is the ability to do work. SI unit: joule (J). Energy takes many forms: kinetic (motion), gravitational potential (height), elastic, chemical, thermal, electrical, light, sound.
Kinetic and potential energy
- Kinetic energy (KE) of moving object: KE = 1/2 m vยฒ (m in kg, v in m/s).
- Gravitational potential energy (PE) at height h: PE = m g h (g โ 9.8 m/sยฒ).
4. Work and Power
Work done (W) when a force moves an object through a distance in the direction of the force:
W = F ร d
Units: N ร m = J (joule).
Power shows how fast work is done:
P = W / t
Units: J / s = W (watt).
5. Conservation of energy
Energy cannot be created or destroyed, only transformed from one form to another. Total energy of an isolated system stays constant.
Kenyan context example (short): In a hydroelectric plant water at height (PE) โ flows down โ turns turbines (KE) โ electrical energy for homes and businesses.
6. Worked examples
Solution: KE = 1/2 m vยฒ = 0.5 ร 800 ร 10ยฒ = 0.5 ร 800 ร 100 = 40,000 J.
Solution: W = Fd = 50 ร 6 = 300 J.
Solution: PE = mgh = 2 ร 9.8 ร 3 = 58.8 J (โ 59 J).
7. Short revision questions (try these)
- Define force and give two examples found at home.
- What is the SI unit of energy? Give an example of 1 J in everyday terms.
- Calculate the work done when a force of 30 N moves an object through 5 m in the force direction.
- Explain with a diagram how energy transforms when a stone is thrown upwards and then falls back down.
- State the law of conservation of energy and relate it to electric power from a dam.
8. Practical notes & classroom checks
- When measuring force use a spring balance (read in newtons); ensure scale calibration is correct.
- When solving problems, keep units consistent (kg, m, s). Convert where needed (e.g., cm โ m).
- Sketch simple force diagrams: show direction and relative sizes of forces with arrows.
- Estimate quickly using g โ 10 m/sยฒ for easier mental calculations; use 9.8 for precision.
9. Quick summary
- Force changes motion; measured in newtons (N).
- Energy is the ability to do work; measured in joules (J).
- Work = force ร distance. Power = work รท time.
- Energy transforms but total energy remains constant in a closed system.