Level: JHS, Basic 8 | Subject: Integrated Science | Strand: Forces and Energy | Topic: Force and Motion
Curriculum reference: NaCCA Basic 8 Integrated Science, Forces and Energy. These notes cover the standard scope of the topic and are checked against the GES General Science learner materials on force.
1. What Is a Force?
A force is a push or a pull. It can change the speed, direction or shape of an object. Force has size and direction. Its unit is the newton (N).
Effects of a force: it can start an object moving, stop it, speed it up, slow it down, change its direction or change its shape.
2. Types of Force
| Force | Meaning and example |
|---|---|
| Friction | Resists motion between two surfaces in contact. Shoes gripping the ground. |
| Gravity (weight) | The pull between objects with mass. It pulls everything toward the Earth. |
| Tension | The pull in a stretched rope or string. |
| Upthrust | The upward push of water on a floating object. |
| Cohesion | Attraction between molecules of the same substance, such as water drops sticking together. |
| Adhesion | Attraction between different substances, such as water sticking to glass. |
| Magnetic and electrostatic | Forces that can act without touching. |
Weight = mass x g, where g is about 10 N/kg on Earth. A 5 kg bag has a weight of 50 N. Mass is in kilograms. Weight is in newtons.
3. Friction
Useful friction: walking without slipping, brakes stopping a car, tyre treads gripping the road, holding a pen.
Harmful friction: wears out machine parts, produces unwanted heat, slows moving objects.
Reducing friction: oil or grease, polishing, ball bearings, wheels.
Increasing friction: rough soles, rough tyres, sprinkling sand on a slippery path.
A block slides further on a smooth surface than on a rough one because the rough surface has more friction.
4. Describing Motion
| Quantity | Meaning | Formula and unit |
|---|---|---|
| Distance | Total path covered (scalar) | metre (m) |
| Displacement | Distance in a stated direction (vector) | metre (m) |
| Speed | How fast an object moves (scalar) | speed = distance / time, m/s |
| Velocity | Speed in a given direction (vector) | velocity = displacement / time, m/s |
| Acceleration | Rate of change of velocity | a = (v – u) / t, m/s squared |
Here u is the starting velocity, v is the final velocity and t is the time. A negative acceleration is called deceleration (slowing down).
5. Newton’s Laws of Motion
- First law: An object stays at rest or moves at constant velocity unless a resultant (net) force acts on it. This is why passengers lean forward when a trotro brakes suddenly.
- Second law: Resultant force = mass x acceleration (F = m x a). A bigger force gives a bigger acceleration. A bigger mass gives a smaller acceleration.
- Third law: For every action there is an equal and opposite reaction. A swimmer pushes the water backward and the water pushes the swimmer forward.
6. Worked Examples
Speed. A runner covers 200 m in 20 s. Speed = 200 / 20 = 10 m/s.
Speed in km/h. A car travels 300 km in 5 hours. Speed = 300 / 5 = 60 km/h.
Acceleration. A car goes from 0 to 20 m/s in 5 s. a = (20 – 0) / 5 = 4 m/s squared.
Force. A car of mass 1500 kg accelerates at 2 m/s squared. F = m x a = 1500 x 2 = 3000 N.
Weight. A girl has a mass of 40 kg. Weight = 40 x 10 = 400 N.
7. Free Fall
Objects of different masses fall at the same rate when air resistance is small. A stone and a heavier stone dropped from the same height hit the ground together. A feather falls slowly only because of air resistance.
8. Forces in Daily Life
- Friction: gripping, writing, braking.
- Push and pull: opening doors, pulling a cart.
- Gravity: falling fruits, rain, planets in orbit.
- Capillarity (adhesion and cohesion): drinking with a straw, water rising in a tissue.
Remember: Speed has no direction but velocity has. Always write the formula, substitute with units, and state the answer with its unit.
Ready to practise?
Education Always (education.inspiringalways.com). Original notes written to follow the NaCCA Basic 8 Integrated Science curriculum.