PHYSICS · Stage 8 · Chapter 6: Forces, Motion and Pressure
Physics 6.19 Chapter 6 Review: Forces, Motion and Pressure
5 min read 6 parts Lesson 19 of 19
Lesson summary
Physics 6.19 Chapter 6 Review: Forces, Motion and Pressure — Alhussan International Academy — Physics (Stages 7–9) notes: Summarise the key ideas of Chapter 6 on speed, forces, stretching, moments, stability, pressure and diffusion.Speed = distance ÷ time; average speed = total d…
- Acceleration
- Distance-time graph
- Speed-time graph
- Accuracy and precision
- Balanced forces
- Ideas about motion
Section 1 of 6Speed, acceleration and graphs
- Speed = distance ÷ time; average speed = total distance ÷ total time.
- Acceleration = change in speed ÷ time, in m/s²; a negative acceleration is deceleration.
- Distance-time graph
- The gradient is the speed and a flat line means stationary.
- Speed-time graph
- The gradient is the acceleration and the area under the line is the distance travelled.
- Accuracy and precision
- Accuracy is closeness to the true value and precision is the number of significant figures; reaction time is about 0.2 s.
Section 2 of 6Forces and motion
- Balanced forces
- Zero resultant force means a stationary object stays still and a moving object keeps a steady speed.
- Unbalanced forces
- A resultant force changes the speed or direction of motion.
- Friction
- It opposes motion, causes wear and heating, and is useful for grip and braking; lubrication and ball bearings reduce it.
- Ideas about motion
- Aristotle thought a force is needed to keep an object moving, Galileo used a thought experiment to show that no force is needed without friction,
- and Newton's first law states this.
- Aristotle thought a force is needed to keep an object moving, Galileo used a thought experiment to show that no force is needed without friction,
Section 3 of 6Stretching, levers and moments
- Hooke's law
- The extension of a spring is proportional to the force up to the elastic limit.
- Spring investigation
- A straight line through the origin shows proportionality, and a point off the line is anomalous and should be checked or repeated.
- Upthrust
- An object floats when upthrust equals its weight.
- Levers
- A lever turns about a pivot, and may multiply force or distance.
- Moments
- Moment = force × perpendicular distance from the pivot, and in equilibrium clockwise moments equal anticlockwise moments.
Section 4 of 6Stability
- Centre of mass
- The point through which the weight of an object seems to act.
- Toppling
- An object topples when the vertical line through its centre of mass falls outside its base.
- More stable
- A low centre of mass and a wide base increase stability.
Section 5 of 6Pressure
- Solids
- Pressure = force ÷ area in pascals (Pa = N/m²), so large areas give low pressure and small areas give high pressure.
- Liquids
- Pressure increases with depth and density and acts in all directions.
- (pressure = depth × density × g)
- In a liquid of the same depth, the denser liquid gives the greater pressure at the bottom.
- Pressure increases with depth and density and acts in all directions.
- Gases
- Pressure is caused by particles colliding with the container walls, and atmospheric pressure decreases with height.
- Weight
- Weight = mass × gravitational field strength.
- (g about 10 N/kg)
- Weight = mass × gravitational field strength.
Section 6 of 6Diffusion
- Diffusion
- It is the random movement and mixing of particles.
- Rate
- It is faster in gases than in liquids and faster at higher temperatures.
| Equation | Quantities and units |
|---|---|
| speed = distance ÷ time | m/s |
| acceleration = change in speed ÷ time | m/s² |
| moment = force × distance | Nm |
| pressure = force ÷ area | Pa or N/m² |
| weight = mass × g | N |
Last updated: