Summary of Motion and Relativity

Overview

This lecture explains all the main concepts, formulas, graphs, and exam questions of the Motion chapter in simple language.

Motion and Relativity

• Motion means the change in position of an object over time.
• Motion is relative; the same object can be moving for one person and stationary for another.

Scalar and Vector Quantities

• Scalar quantities have only magnitude, no direction.
• Vectors have both magnitude and direction.
• Example: Distance is scalar, displacement is vector.

Distance and Displacement

• Distance: The total path traveled by the object, always positive.
• Displacement: The shortest distance between the initial and final points; it can be zero or negative.
• Distance ≥ Displacement

Speed and Velocity

• Speed = total distance / total time; units: m/s or km/h; it is scalar.
• Velocity = displacement / time; it is a vector quantity.
• To convert speed to m/s: km/h × (5/18)
• Average speed = total distance / total time
• Average velocity = (initial velocity + final velocity) / 2 (if acceleration is uniform)

Acceleration and Retardation

• Acceleration = (final velocity – initial velocity) / time
• Unit: m/s²
• Acceleration is positive when speed increases, negative (retardation) when it decreases.
• If speed is uniform, acceleration is zero.

Uniform and Non-Uniform Motion

• Uniform motion: equal distances covered in equal intervals of time, constant speed.
• Non-uniform: speed/velocity changes, acceleration occurs.

Equation of Motion

• v = u + at
• s = ut + ½ at²
• v² = u² + 2as

Free Fall and Gravity

• In free fall, acceleration = g = 10 m/s² (or 9.8)
• g is negative when moving downward, positive when moving upward.

Graphs

• Distance-Time graph: slope = speed, straight line = uniform motion
• Velocity-Time graph: slope = acceleration, area under graph = distance/displacement

Key Terms & Definitions

• Motion — change in position of an object over time.
• Distance — total path traveled by the object, scalar.
• Displacement — shortest straight distance between start and end points, vector.
• Speed — distance/time; no direction.
• Velocity — displacement/time; with direction.
• Acceleration — change in velocity per second.
• Retardation — decrease in speed; negative acceleration.
• Uniform Motion — equal distance in equal time.
• Non-Uniform Motion — motion with changing speed/acceleration.

Action Items / Next Steps

• Homework: Solve questions related to free fall.
• Practice graph-based questions.
• Mention the next chapter in the comments.