Class 9 Physics Chapter 1 Motion

 CLASS 9 PHYSICS – MOTION

1. Introduction to Motion

Motion is the change in position of an object with respect to time. Everything we observe around us—cars, planets, falling objects—exhibits motion in some form.

If an object does not change its position over time, it is considered to be at rest.

2. Types of Motion

a) Linear Motion

Motion along a straight path.
Example: A car moving on a straight highway.

b) Circular Motion

Motion along a circular path.
Example: A ceiling fan blade.

c) Periodic/Oscillatory Motion

Motion that repeats at regular intervals.
Example: A swinging pendulum.

d) Rotational Motion

Object rotates about its own axis.
Example: Earth rotating around its axis.

3. Distance and Displacement

Distance

• Total path travelled

• Scalar quantity

• Always positive

Displacement

• Shortest straight-line distance between start and end point

• Vector quantity

• Can be positive, zero, or negative

Example:
Walking 5 m forward, then 5 m back →

• Distance = 10 m

• Displacement = 0 m

4. Speed, Velocity, and Acceleration

Speed

$$\text{Speed} = \frac{\text{Distance}}{\text{Time}}$$

Scalar quantity.

Velocity

$$\text{Velocity} = \frac{\text{Displacement}}{\text{Time}}$$

Vector quantity (direction matters).

Acceleration

$$a = \frac{v - u}{t}$$

Rate of change of velocity.

5. Uniform & Non-Uniform Motion

                                        

Uniform Motion

Equal distances in equal time intervals.
Example: A train moving steadily at 60 km/h.

Non-Uniform Motion

Unequal distances in equal time intervals.
Example: A bike moving in traffic.

6. Graphs of Motion

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a) Distance–Time Graph

• Straight line → uniform motion

• Curved line → non-uniform motion

• Horizontal line → at rest

• Slope = speed

b) Velocity–Time Graph

• Slope = acceleration

• Area under graph = distance travelled

7. Equations of Motion (Uniform Acceleration)

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1. $v = u + at$

2. $s = ut + \frac{1}{2}at^2$

3. $v^2 = u^2 + 2as$

Where:

• $u$ = initial velocity

• $v$ = final velocity

• $a$ = acceleration

• $t$ = time

• $s$ = displacement

8. Free Fall

Objects falling under gravity alone experience uniform acceleration.

$$g = 9.8 \, m/s^2$$

Direction: downward.

9. Relative Motion

Relative motion describes how one object appears to move from the perspective of another object.

Example: Two cars moving side-by-side at the same speed appear stationary relative to each other.

10. Summary

• Motion = change in position

• Distance → scalar; Displacement → vector

• Speed → scalar; Velocity → vector

• Acceleration = change in velocity/time

• Graphs help visualize motion

• Equations of motion apply only when acceleration is constant