Class IX SEBA Science – Chapter 8: Motion
🌀 1. Motion
- An object is said to be in motion if it changes its position with time.
- Rest: No change in position over time with respect to surroundings.
- Types of motion:
- Translatory motion: Straight line
- Circular motion: Around a fixed point
- Oscillatory motion: To-and-fro (like a pendulum)
📏 2. Distance and Displacement
| Quantity | Distance | Displacement |
|---|---|---|
| Type | Scalar | Vector |
| Meaning | Total path covered | Shortest path from start to end |
| Value | Always positive | Can be positive, zero, or negative |
| Example | 10 m to-and-fro walk → 20 m | 10 m to-and-fro walk → 0 m |
🕐 3. Uniform and Non-uniform Motion
- Uniform motion: Equal distance in equal intervals of time (e.g., car at constant speed).
- Non-uniform motion: Unequal distance in equal intervals of time (e.g., moving in traffic).
🔣 4. Speed and Velocity
- Speed = Distance / Time → scalar quantity
- Velocity = Displacement / Time → vector quantity
- Uniform speed: Constant speed
- Average speed = Total distance / Total time
- Average velocity = Total displacement / Total time
📉 5. Acceleration
- Acceleration (a) = Change in velocity / Time taken
- Unit: m/s²
- Can be:
- Positive (speeding up)
- Negative (slowing down → called retardation)
📐 6. Graphical Representation of Motion
a) Distance-Time Graph
- Straight line → Uniform speed
- Curved line → Non-uniform speed
b) Velocity-Time Graph
- Area under graph → Gives displacement
- Straight horizontal line → Constant velocity
- Sloped line → Uniform acceleration
📊 Sample Graph Shapes:
- Velocity-time graph for uniform acceleration: Straight upward sloped line
- Velocity-time graph for retardation: Downward sloped line
🧮 7. Equations of Motion (for Uniform Acceleration)
Let:
- Initial velocity = u
- Final velocity = v
- Acceleration = a
- Time = t
- Displacement = s
Equations:
- v = u + at
- s = ut + ½at²
- v² = u² + 2as
These can be derived using graphical method.
🚀 8. Uniform Circular Motion
- An object moving in a circle at constant speed has uniform circular motion.
- Direction changes continuously → hence it’s accelerated motion.
- Velocity changes though speed is constant.
📘 Exercise Questions and Answers
1. An object has moved through a distance. Can it have zero displacement?
Yes, if it returns to the starting point (like a circular path), displacement = 0.
2. What do you mean by uniform and non-uniform motion?
- Uniform: Equal distance in equal time
- Non-uniform: Unequal distance in equal time
3. What does the odometer of an automobile measure?
It measures the distance travelled by the vehicle.
4. What does the path of an object look like in uniform motion?
It is a straight line.
5. During a thunderstorm, a thunder is heard 10 seconds after lightning is seen. How far is the storm? (Speed of sound = 340 m/s)
Distance = Speed × Time = 340 × 10 = 3400 m = 3.4 km
6. Derive the equation: v² = u² + 2as
Using:
- v = u + at
- s = ut + ½at²
⇒ Eliminate time (t) to derive:
v² = u² + 2as
✍️ Extra Important Questions
1. Define acceleration.
Rate of change of velocity.
a = (v – u)/t
2. What is the displacement of an object moving in a circular path of radius 7 m after half a revolution?
Displacement = Diameter = 2 × 7 = 14 m
3. Differentiate between speed and velocity.
- Speed → Scalar, only magnitude
- Velocity → Vector, magnitude + direction
4. A car travels 60 km in 2 hours. Find its average speed.
Speed = Distance / Time = 60 / 2 = 30 km/h
⭐ Points to Remember
- Distance is always ≥ displacement
- Speed is scalar; velocity is vector
- Acceleration can be negative (retardation)
- Uniform motion = straight-line graph
- Area under v-t graph = displacement
- Circular motion = constant speed but changing velocity (acceleration exists)
✅ MCQs for Practice
1. What is the SI unit of speed?
a) m
b) m/s ✅
c) km/hr
d) m/s²
2. Which graph shows uniform motion?
a) Curved distance-time graph
b) Sloped straight line in distance-time graph ✅
c) Horizontal line in v-t graph
d) Parabolic v-t graph
3. If a body is moving in a circle with constant speed, then:
a) Velocity is constant
b) No acceleration
c) Velocity changes due to change in direction ✅
d) Displacement is zero
4. Which of these is a vector quantity?
a) Speed
b) Distance
c) Velocity ✅
d) Time
proceed to Chapter 9: Force and Laws of Motion?
