Introduction to Motion: Physics for Class 9
Everything Moves
The wind moves, the planets move, and even the electrons in the screen you are looking at are moving. Motion is simply a change in position of an object over time.
Scalars vs. Vectors
This is the first "Physics" concept you must master.
Scalar Quantity
Has only Magnitude (Size).
- Distance (5m)
- Speed (10 m/s)
- Time (5s)
Vector Quantity
Has both Magnitude AND Direction.
- Displacement (5m North)
- Velocity (10 m/s East)
- Acceleration
Distance vs. Displacement
The Classic Example
Imagine you walk 5m East, then turn back and walk 5m West.
Distance: 5 + 5 = 10m (Total path covered).
Displacement: 5 - 5 = 0m (Shortest path from start to end).
The Three Equations of Motion
Applicable only for Uniform Acceleration.
v = u + at
s = ut + ½at²
2as = v² - u²
where u=initial velocity, v=final velocity, a=acceleration, s=displacement, t=time.
Exam Focus & NCERT
1
Numerical Problems (3-5 Marks)
"A car starts from rest (u=0) and accelerates at 5m/s². Find distance covered in 10s." (Use 2nd Equation).
2
Graph Questions (3 Marks)
"Calculate the distance covered by finding the Area under the Velocity-Time graph."
Practice Questions
Q1. Conceptual
Can displacement be zero while distance is not zero?
Q2. Numerical
A bus decreases speed from 80 km/h to 60 km/h in 5s. Find acceleration.
Hint: Convert km/h to m/s by multiplying with 5/18!
Conclusion
Motion is intuitive—we see it every day. The trick is to translate what you see into numbers (Scalars/Vectors) and formulas. Don't fear the Numericals; they are your path to full marks!
Physics Made Simple
Get step-by-step solutions to all NCERT numericals in our notes section.