Motion Notes: Class 9 Science Chapter 7
Class 9 Science Chapter 7 Motion covers concepts of rest and motion, distance, displacement, speed, velocity, acceleration, distance-time and velocity-time graphs, equations of motion, and uniform circular motion.
Overview of Topics & Subtopics (Click to Jump)
1. Describing Motion, Distance & Displacement
Short Note:
• State of Rest and Motion: An object is in motion if its position changes relative to a reference point with time.
• Distance: The total length of the path travelled by an object. It is a scalar quantity and always positive.
• Displacement: The shortest straight-line distance from the initial position to the final position. It is a vector quantity and can be positive, negative, or zero.
• Key Point: Displacement can be zero even if distance is non-zero (e.g., returning to the starting point in a closed path).
2. Speed, Velocity & Acceleration
Short Note:
• Speed: Distance travelled per unit time (m/s). Scalar quantity. Average Speed = Total Distance / Total Time.
• Velocity: Displacement per unit time in a specified direction (m/s). Vector quantity. Average Velocity = (Initial Velocity + Final Velocity) / 2 = (u + v) / 2.
• Acceleration (a): Rate of change of velocity with time. SI Unit: m/s².
- Formula: a = (v - u) / t where u = initial velocity, v = final velocity, t = time.
- Positive acceleration: Velocity increases with time.
- Negative acceleration (Retardation/Deceleration): Velocity decreases with time.
3. Graphical Representation & Equations of Motion
Short Note:
• Distance-Time Graph: Slope gives the speed of the object.
• Velocity-Time Graph: Slope gives the acceleration, and the area under the velocity-time graph gives the displacement (or distance).
• Equations of Motion (for uniform acceleration):
1. First Equation: v = u + at (Velocity-time relation)
2. Second Equation: s = ut + ½ at² (Position-time relation)
3. Third Equation: v² = u² + 2as (Position-velocity relation)
4. Uniform Circular Motion
Short Note:
• Definition: When an object moves in a circular path with constant speed, its motion is called uniform circular motion.
• Accelerated Motion: Even though the speed remains constant, the direction of motion changes continuously at every point, making uniform circular motion an accelerated motion.
• Formula for Speed: v = (2πr) / t, where r is the radius of the circular path and t is the time taken for one full revolution.
Intext Questions - Page Number: 74
Example: If an athlete runs around a circular track of radius r and returns back to the starting point, the distance covered is 2πr, but the displacement is zero.
• Side of square field = 10 m; Perimeter = 4 × 10 = 40 m.
• Time taken to cover 40 m = 40 s (1 round = 40 s).
• Total time given = 2 min 20 s = 140 s.
• Number of rounds in 140 s = 140 / 40 = 3.5 rounds.
• After 3 complete rounds, the farmer returns to the starting corner. In the remaining 0.5 round, he reaches the diagonally opposite corner.
• Displacement = Diagonal of square field = √(10² + 10²) = √(200) = 10√2 m ≈ 14.14 m.
Intext Questions - Page Number: 76
1. Nature: Speed is a scalar quantity (has magnitude only), whereas velocity is a vector quantity (has both magnitude and direction).
2. Formula: Speed = Distance / Time, whereas Velocity = Displacement / Time.
3. Value: Speed is always positive or zero, whereas Velocity can be positive, negative, or zero.
Textbook Chapter End Exercise Questions
• Initial velocity (u) = 80 km/h = 80 × (5/18) m/s = 22.22 m/s
• Final velocity (v) = 60 km/h = 60 × (5/18) m/s = 16.67 m/s
• Time (t) = 5 s
• Acceleration (a) = (v - u) / t = (16.67 - 22.22) / 5 = -5.55 / 5 = -1.11 m/s²
(Negative sign indicates retardation).
1. v = u + at
2. s = ut + ½ at²
3. v² = u² + 2as
Where: u = initial velocity, v = final velocity, a = uniform acceleration, t = time taken, and s = distance/displacement covered.