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1. Describing Motion Scientific Units in Physics Speed, Distance, and Time Calculating Speed Relative Motion (cars, trains passing) Forces as Pushes and Pulls Measuring Forces (Newtons, springs) Balanced and Unbalanced Forces Force Diagrams and Arrows Effect of Forces on Motion Equilibrium and Opposing Forces Hooke’s Law (Introductory) Friction and Air/Water Resistance
Year 8 • Physics
Describing Motion & Forces
This lesson explains how motion is described using scientific units, how speed is calculated, how forces act as pushes and pulls, and how forces affect motion, equilibrium, and shape.
Lesson Objectives
- Describe motion using speed, distance, and time.
- Calculate speed using the speed equation.
- Explain relative motion with real-life examples.
- Understand forces as pushes and pulls.
- Explain balanced and unbalanced forces.
- Describe friction, air resistance, and Hooke’s Law.
1. Scientific Units in Physics
Physics uses standard scientific units so measurements are the same everywhere.
Common Units:
• Distance – metres (m)
• Time – seconds (s)
• Speed – metres per second (m/s)
• Force – newtons (N)
• Distance – metres (m)
• Time – seconds (s)
• Speed – metres per second (m/s)
• Force – newtons (N)
2. Speed, Distance, and Time
Speed tells us how fast something is moving.
Speed Formula:
Speed = Distance ÷ Time
Speed = Distance ÷ Time
Example: A car travels 100 m in 5 s.
Speed = 100 ÷ 5 = 20 m/s
3. Relative Motion
Relative motion means motion depends on the observer.
Examples:
• A passenger sitting in a moving train is still relative to the train
• Two cars moving side by side at the same speed appear stationary to each other
• A faster train overtaking a slower train appears to move quickly
• A passenger sitting in a moving train is still relative to the train
• Two cars moving side by side at the same speed appear stationary to each other
• A faster train overtaking a slower train appears to move quickly
4. Forces as Pushes and Pulls
A force is a push or a pull that can change motion or shape.
Examples:
• Pushing a door
• Pulling a trolley
• Gravity pulling objects downward
• Pushing a door
• Pulling a trolley
• Gravity pulling objects downward
5. Measuring Forces
Forces are measured using a spring balance and measured in newtons (N).
A bigger force stretches the spring more.
6. Balanced and Unbalanced Forces
Balanced Forces: Equal forces, no change in motion
Unbalanced Forces: Unequal forces, motion changes
Unbalanced Forces: Unequal forces, motion changes
7. Force Diagrams and Arrows
Forces are shown using arrows.
• Arrow direction shows force direction
• Arrow length shows force size
• Arrow length shows force size
8. Effect of Forces on Motion
Forces can:
• Start motion
• Stop motion
• Change speed
• Change direction
• Change shape
• Start motion
• Stop motion
• Change speed
• Change direction
• Change shape
9. Hooke’s Law (Introductory)
Hooke’s Law states that the extension of a spring is proportional to the force applied (within limits).
More force → more stretch (up to the elastic limit)
10. Friction and Air/Water Resistance
• Friction slows moving objects
• Air resistance slows falling objects
• Water resistance slows swimmers and boats
• Air resistance slows falling objects
• Water resistance slows swimmers and boats
Practice Questions
A. Fill in the Blanks
- Speed is measured in __________.
- Force is measured in __________.
- Speed = distance ÷ __________.
- Forces are pushes or __________.
- A stretched spring stores __________ energy.
B. True or False
- Balanced forces change motion.
- Friction slows objects down.
- Speed uses metres and seconds.
- Air resistance increases with speed.
- A longer arrow shows a larger force.
C. Short Answer
- Define speed.
- What is relative motion?
- Name two contact forces.
- What does a force arrow show?
- State Hooke’s Law.
D. Numerical
- A cyclist travels 200 m in 20 s. Calculate speed.
- A car travels 600 m in 30 s. Find speed.
- A runner moves 100 m in 10 s. Calculate speed.
- How far does an object travel in 5 s at 4 m/s?
- How long to travel 90 m at 3 m/s?
✅ Show Answer Key
A. Fill in the Blanks
- m/s
- newtons
- time
- pulls
- elastic potential
B. True or False
- False
- True
- True
- True
- True
C. Short Answer
- Speed is distance travelled per unit time.
- Motion compared to another object.
- Friction, air resistance.
- Direction and size of force.
- Extension is proportional to force.
D. Numerical
- 10 m/s
- 20 m/s
- 10 m/s
- 20 m
- 30 s
© Aviate Learning – Describing Motion & Forces (Year 8)