Force, Pressure and Friction: Class 8 Study Guide
Forces change motion and shape. Pressure and friction explain many everyday events.
A force is a push or a pull. It has a size (magnitude) and a direction, and is measured in newtons (N). Forces can change an object's speed, direction or shape. Contact forces need touching (muscular force, friction). Non-contact forces act at a distance (gravity, magnetism, electrostatic force). When forces are balanced, there is no change in motion; unbalanced forces change it.
Pressure is the force acting on a unit area: pressure = force ÷ area. A sharp knife cuts better than a blunt one because the same force acts on a smaller area. Pressure is measured in pascals (Pa), where 1 Pa = 1 N/m².
Pressure in everyday life
Camels have broad feet that spread their weight and stop them sinking in sand. Nails have pointed tips to concentrate force. Liquids exert pressure on the base and walls of a container, and the pressure increases with depth, which is why dams are thicker at the bottom. The atmosphere also exerts pressure on us (atmospheric pressure), which we do not feel because the pressure inside our bodies balances it.
| Situation | Force | Area | Pressure |
|---|---|---|---|
| Sharp knife | 20 N | 0.0001 m² | 200 000 Pa |
| Blunt knife | 20 N | 0.001 m² | 20 000 Pa |
| Standing on both feet | 600 N | 0.04 m² | 15 000 Pa |
| Standing on one foot | 600 N | 0.02 m² | 30 000 Pa |
Friction
Friction is the force that opposes motion between surfaces in contact. It arises from surface roughness. It is helpful (grip of shoes, brakes, writing with a pencil) and harmful (wear of machine parts, heat). Friction can be increased by making surfaces rougher (treads on tyres) and reduced by lubricants (oil), polishing, ball bearings or by streamlining in air and water. Rolling friction is less than sliding friction, which is why wheels are so useful.
Carpet, tile, sandpaper.
Same push each time.
Rougher means shorter.
More friction slows objects.
Types of friction
Static friction stops objects from starting to move. Sliding friction acts when one surface slides over another. Rolling friction acts on rolling objects and is smaller. Fluid friction (drag) acts on objects moving through liquids and gases, and it is reduced by a streamlined shape.
Worked example and common misconceptions
Example: A force of 100 N on an area of 0.5 m² gives a pressure of 200 Pa. Common misconceptions: thinking friction is always bad, that heavier objects always have more friction than lighter ones on the same surface only because of their weight (it also depends on the surfaces) and that pressure and force are the same. Safety: do not test pressure with sharp objects.
Check your understanding: model answers
1. 250 Pa. 2. Wide tyres spread the weight over a larger area and reduce pressure on the soil. 3. For example, using oil or ball bearings.
✨ Key Takeaways
- Force is measured in newtons.
- Pressure = force ÷ area.
- Friction opposes motion.
- Lubricants and wheels reduce friction.
💬 Questions for Discussion & Reflection
- Find the pressure: 500 N on 2 m².
- Why do tractors have wide tyres?
- Name one way to reduce friction.
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