Distance and Displacement: Class 9 Science (Chapter 8 – Motion)

Understanding distance and displacement is a key concept in CBSE Class 9 Science (Chapter 7 – Motion). While both terms describe how far an object moves, they mean very different things. Let’s break it down with clear examples and easy explanations!

What is Distance?

Distance is the total path length traveled by an object, no matter its direction. It’s a scalar quantity, meaning it only has magnitude (size) and no direction.

Example of Distance:

Imagine you walk 5 meters east, then 3 meters west.

• Total distance traveled = 5 m + 3 m = 8 m

Even though you ended up only 2 meters from your starting point, the distance you walked is still 8 meters.

What is Displacement?

Displacement is the shortest straight-line distance between the starting and ending points, including direction. It’s a vector quantity, meaning it has both magnitude and direction.

Example of Displacement:

Using the same scenario:

• You walk 5 meters east, then 3 meters west.
• Displacement = Final position – Initial position = 2 meters east

Your displacement is 2 meters east because that’s how far you are from where you started, in a straight line.

Difference Between Distance and Displacement

Aspect	Distance	Displacement
Definition	Total path length traveled	Shortest path from start to end
Quantity Type	Scalar (only magnitude)	Vector (magnitude + direction)
Can Be Zero?	No (always positive)	Yes (if start and end points match)
Example A	Runner completing a lap (400 m)	Runner ends at start (0 m disp.)
Example B	Runner in a race (200 m)	Runner finishes 200m from start

Real-World Examples of Distance and Displacement

1. Circular Track

• An athlete runs one full lap around a 200 m circular track.
  • Distance = 200 m (total path)
  • Displacement = 0 m (back to the start)

2. Road Trip

• You drive 60 km north, then 25 km south.
  • Distance = 60 + 25 = 85 km
  • Displacement = 35 km north

3. Farmer’s Field

• A farmer walks along the boundary of a 10 m × 10 m square field once.
  • Distance = 40 m (perimeter)
  • Displacement = 0 m (returns to start)

When are Distance and Displacement the same?

Only when an object moves in a straight line without changing direction.

• Example: A car drives 50 km straight east.
  • Distance = 50 km
  • Displacement = 50 km east

Example from the Textbook (Page 2, Fig. 7.1)

The textbook describes an object moving along a straight path:

• Starts at point O (reference point).
• Moves to A (60 km away), then back to B (25 km from O), and finally to C (35 km from O).

Calculating Distance (Total Path Length)

• From O → A → C:
  • Distance = OA + AC = 60 km + 35 km = 95 km
• If the object returns to O:
  • Distance = OA + AO = 60 km + 60 km = 120 km

🔹 Key Point: Distance only cares about how much ground was covered, not direction.

Calculating Displacement (Net Position Change)

• From O → A → C:
  • Displacement = Final position – Initial position = 35 km from O
• If the object returns to O:
  • Displacement = 0 km (since it ends where it started)

🔹 Key Point: Displacement is the shortest straight-line distance from start to finish with directio

Remember:

• Distance is like a car’s odometer—it adds up every meter traveled.
• Displacement is like a GPS arrow—it tells you how far (and in what direction) you are from the start.

Go back to NCERT Science Class 9,  for crystal-clear concepts, solved examples, and practice questions! 📖✨ #LearnWithNCERTguru”