Motion
1. Rest and Motion
Rest
A body is said to be at REST if it does NOT change its position with respect to its surroundings over time.
Examples: A book on a table, a parked car.
Motion
A body is said to be in MOTION if it CHANGES its position with respect to its surroundings over time.
Examples: A running car, a flying bird, flowing water.
Relative Nature
Rest and motion are RELATIVE. A passenger sitting in a moving train is at rest relative to the train but in motion relative to the ground.
2. Types of Motion
Based on Path
| Type | Definition | Example |
|---|---|---|
| Translatory | Body moves as a WHOLE along a line | A car on a straight road |
| - Rectilinear | Straight line path | A falling stone |
| - Curvilinear | Curved path | A thrown ball |
| Rotatory (Circular) | Body moves around a fixed axis | Earth's rotation, a spinning top |
| Periodic | Motion that REPEATS after a fixed time | Earth's revolution, pendulum |
| Oscillatory | To-and-fro motion about a mean position | Swing, pendulum, vibrating string |
| Random | No fixed path | A flying butterfly, gas molecules |
Combination of Motions
Many objects have COMBINATION motion. Example: a rolling ball has both TRANSLATORY (forward) and ROTATORY (spinning) motion.
3. Speed
Speed = Distance / Time
Units
- SI: metre/second (m/s)
- Common: kilometre/hour (km/h), cm/s
Conversion
1 km/h = 1000 m / 3600 s = 5/18 m/s 1 m/s = 18/5 km/h
Uniform vs Non-Uniform Speed
| Type | Definition | Example |
|---|---|---|
| Uniform speed | Equal distance in equal time intervals | Car at constant 40 km/h |
| Non-uniform speed | Unequal distance in equal time intervals | Car in city traffic |
4. Distance-Time Graph
A DISTANCE-TIME graph shows how distance changes over time.
Reading Distance-Time Graphs
- Straight line sloping upward: UNIFORM speed. Steeper slope = higher speed.
- Horizontal line: Body is at REST (no movement).
- Curved line: NON-UNIFORM speed.
- Downward slope: Body is moving BACK towards start.
Finding Speed from Graph
Speed = Slope = (Change in distance) / (Change in time)
Worked Example (ICSE 2024, 3 marks)
'A body travels 20 m in 5 s, then rests for 3 s, then travels 30 m in 5 s. Draw distance-time graph and find speed in first part.'
Solution: Speed in first part = 20/5 = 4 m/s. Graph: Straight line from (0,0) to (5,20), horizontal from (5,20) to (8,20), then straight to (13,50).
5. Speed and Velocity (Basic Comparison)
| Quantity | Definition | Type |
|---|---|---|
| Speed | Distance / Time | Scalar (only magnitude) |
| Velocity | Displacement / Time | Vector (magnitude + direction) |
At this level, we focus mainly on SPEED.
6. Average Speed
When speed is NOT constant: Average speed = Total distance travelled / Total time taken
Example (ICSE 2023, 3 marks)
'A car travels 60 km at 30 km/h and then 60 km at 60 km/h. Find average speed.'
Solution: Time for first part = 60/30 = 2 h. Time for second part = 60/60 = 1 h. Total distance = 120 km. Total time = 3 h. Average speed = 120/3 = 40 km/h.
7. ICSE Exam Focus
| Topic | Marks | Frequency |
|---|---|---|
| Types of motion (identify) | 2 marks | Very High |
| Speed calculations | 2-3 marks | Very High |
| Distance-time graphs | 3 marks | High |
| Average speed | 3 marks | High |
| Unit conversions | 1-2 marks | Medium |
Common Mistakes
- Confusing speed and velocity (velocity includes direction).
- Incorrect unit conversion (use 5/18 for km/h to m/s).
- Reading distance-time graph as the path of motion (it is NOT the path — it is a plot of distance vs time).
Self-Test (5 Questions)
Q1. 'A ball thrown vertically upward — what type of motion?' (1 mark)
- A) Periodic
- B) Translatory
- C) Oscillatory
- D) Random
Q2. Convert 72 km/h to m/s. (1 mark)
Q3. 'A train travels 300 km in 5 hours. Find its speed in m/s.' (2 marks)
Q4. 'A car moves 40 km at 20 km/h, then 60 km at 30 km/h. Find average speed.' (3 marks)
Q5. 'What does a horizontal line in a distance-time graph mean?' (1 mark)
Answers
A1. B) Translatory (rectilinear). A2. 20 m/s. (72 × 5/18 = 20 m/s.) A3. Speed = 60 km/h = 60 × 5/18 = 50/3 m/s ≈ 16.67 m/s. A4. Total distance = 100 km. Time₁ = 40/20 = 2 h. Time₂ = 60/30 = 2 h. Total time = 4 h. Average speed = 100/4 = 25 km/h. A5. The body is at REST (no change in distance with time).
