CBSE Class 9 Science Notes Chapter 7 Motion

In the physical world, motion is one of the most prevalent phenomena. The branch of physics that examines the behavior of moving objects is called mechanics. Mechanics is divided into two main areas: Kinematics and Dynamics.

• Kinematics focuses on describing motion, including aspects like displacement, velocity, and acceleration, without considering the forces causing the motion.
• Dynamics examines the forces that cause motion, studying how these forces influence the movement of objects.

1.0Motion and Rest

• Motion-An object is considered to be in motion if it changes its location relative to its surroundings over a given period of time.
• Rest-An object is said to be at rest if its position remains unchanged relative to its surroundings.

Note-Frame of Reference-A frame of reference is an object or coordinate system against which we measure and compare the position of another object.

2.0Types of Motion

1. Translatory Motion-Particles move from one point in space to another. This motion can occur either along a straight line or along a curved path.

Example- A car traveling along a straight road 

2. Rotatory Motion-The particles of the body describe concentric circles about the axis of rotation.

Example- Earth's Rotation: The Earth rotates around its axis, causing day and night.

3. Vibratory Motion-Particles oscillate back and forth around a fixed point.

Example- Pendulum: A pendulum swings back and forth around its equilibrium position.

3.0Distance and Displacement

4.0Uniform and Non-Uniform Motion

Uniform Motion-When a body covers equal distances in equal intervals of time.

Non-Uniform Motion-When a body covers unequal distances in equal intervals of time, or vice versa.

5.0Speed

• Speed is the rate at which an object travels or covers a distance. It is a scalar quantity that indicates how fast an object moves from one point to another, without considering the direction of travel.
• Speed=Distance/Time=S/t
• Speed is a scalar quantity.
• SI Unit is m/s

Uniform Speed

• Uniform speed refers to a situation where an object moves with a constant magnitude of speed.

Non-Uniform Speed

• If an Object covers equal distances in unequal intervals of time or vice-versa.

Uniform Velocity

• Uniform velocity refers to a situation where an object moves in a straight line with both constant speed and constant direction.

Non-Uniform Velocity 

• Non-uniform velocity describes a situation where an object's velocity changes in either magnitude, direction, or both.

6.0Acceleration

• It is defined as the rate at which the velocity of a roaming body changes over time. This change can involve a variation in the object's speed, its direction of motion, or both.
• Acceleration=Rate of change of velocity with time
• Acceleration=Change in velocity/Time
• a=v-ut
• SI Unit-ms2
• Positive Acceleration-If velocity of object increases.
• Negative Acceleration or Retardation-If velocity of object decreases.
• Zero Acceleration-If the change in velocity is zero either the object is at rest or moving with uniform velocity .

7.0Uniform Acceleration

• If the change in velocity in equal intervals of time is always the same, then the object is said to be roaming with uniform acceleration.

Non Uniform acceleration

• If the alter in velocity over equal time intervals is inconsistent, the object is said to be experiencing variable acceleration.