Overview
This lecture covers the fundamental concepts of motion for class 9, including definitions, formulas, types of motion, distinctions between key terms, and practice with important questions and graphs.
Motion and Rest
- Motion refers to a change in the position of an object with respect to time.
- Rest is when an object does not change its position with respect to its surroundings.
- Motion is a relative concept; it depends on the observer’s frame of reference.
Scalars and Vectors
- Scalar quantities have only magnitude (e.g., distance, speed, mass).
- Vector quantities have both magnitude and direction (e.g., displacement, velocity, force).
Distance and Displacement
- Distance: The total length of the path traveled by an object; scalar; always positive.
- Displacement: The shortest, straight-line distance between the initial and final position; vector; can be positive, negative, or zero.
- Displacement formula: Final position – Initial position.
- Distance can never be zero if there is movement; displacement can be zero if the starting and ending positions are the same.
Speed and Velocity
- Speed: Distance traveled per unit time; scalar; Speed = Distance/Time.
- Velocity: Displacement per unit time; vector; Velocity = Displacement/Time.
- If direction is constant, speed and velocity can be numerically equal.
- In circular motion, speed can be constant but velocity is not due to changing direction.
Average Speed and Velocity
- Average speed = Total distance / Total time.
- Average velocity = (Initial velocity + Final velocity) / 2 for constant acceleration.
Acceleration
- Acceleration is the rate of change of velocity per unit time; vector.
- Acceleration = (Final velocity – Initial velocity) / Time.
- Positive acceleration: speeding up; Negative acceleration (retardation): slowing down.
- Unit: meter per second squared (m/s²).
- If velocity doesn’t change, acceleration is zero.
Uniform and Non-Uniform Motion
- Uniform motion: Equal distance in equal time intervals; velocity is constant; acceleration is zero.
- Non-uniform motion: Unequal distance in equal time intervals; velocity or speed changes; acceleration is not zero.
Equations of Motion (for Constant Acceleration)
- v = u + at
- s = ut + ½ at²
- v² = u² + 2as
- Where, v = final velocity, u = initial velocity, a = acceleration, s = displacement, t = time.
Free Fall and Sign Convention
- During free fall, initial velocity (u) is zero if dropped from rest.
- Acceleration due to gravity (g) is taken as negative (–9.8 or –10 m/s²) when directed downward.
- Displacement is negative when moving downward, positive when upward.
Graphs in Motion
- Distance-Time Graph: Slope represents speed.
- Straight line = uniform speed; Curved line = changing speed (acceleration).
- Velocity-Time Graph: Slope represents acceleration; Area under graph = displacement.
- Straight line parallel to time axis = constant velocity (zero acceleration).
- Sloped line = acceleration; Negative slope = deceleration.
Key Terms & Definitions
- Motion — Change in position with respect to time.
- Rest — No change in position with respect to surroundings.
- Distance — Total path traveled (scalar).
- Displacement — Shortest straight-line distance between start and end positions (vector).
- Speed — Rate of covering distance (scalar).
- Velocity — Rate of covering displacement in a direction (vector).
- Acceleration — Rate of change of velocity (vector).
- Uniform motion — Constant speed or velocity.
- Non-uniform motion — Changing speed or velocity.
- Free fall — Motion under gravity with no other force.
- Average speed/velocity — Total distance/displacement by total time.
- Retardation — Negative acceleration.
Action Items / Next Steps
- Practice numerical problems based on equations of motion and graph interpretation.
- Complete the assigned homework question on free fall and share your answer in the comments.
- Review the sign conventions for gravity and displacement in free fall problems.