ICSE 7 Physics Motion Short Notes

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Table of Contents

    ⚡ Fast Revision: Motion Concepts

    Rest vs. Motion
    • Rest: An object is at rest if it does not change its position with respect to a fixed reference point.
    • Motion: A change in position over time relative to surroundings.
    • Relative Nature: Rest and motion are relative; an object can be at rest for one observer and in motion for another.
    Unit Alert

    Distance/Displacement: s | SI Unit: metre (m)

    Speed/Velocity: v | SI Unit: m/s (ms⁻¹)

    Distance Displacement
    Total path length covered. Shortest path between start and end.
    Scalar (No direction). Vector (Has direction).
    Always positive. Can be zero if start = end.
    ❌ Common Error:

    Thinking displacement can be greater than distance. Fix: Distance is always $\ge$ Displacement.

    Distance vs Displacement Visualization

    ⚡ Fast Revision: Types of Motion

    Translatory Motion
    • Rectilinear: Motion along a straight line (e.g., a car on a straight road).
    • Curvilinear: Motion along a curved path (e.g., a ball thrown at an angle).
    • Key Feature: Every part of the object moves the same distance in the same time.
    Motion Type Definition Example
    Rotatory Movement around a fixed axis. Spinning top, Ceiling fan.
    Oscillatory To-and-fro motion about a mean position. Simple pendulum, Swing.
    Periodic Repeats at regular intervals of time. Heartbeat, Phases of Moon.
    The Time Period ($T$):

    $T = \frac{\text{Total Time}}{\text{Number of Oscillations}}$

    Unit: second ($s$)

    ❌ Common Error:

    Confusing Rotatory with Circular motion. Fix: In Rotatory, the axis passes through the object; in Circular, the object moves around an external point.

    Categories of Motion

    ⚡ Fast Revision: Scalar vs. Vector & Uniform Motion

    Uniform vs. Non-Uniform Motion
    • Uniform Motion: Covers equal distances in equal intervals of time. Velocity remains constant.
    • Non-Uniform Motion: Covers unequal distances in equal intervals of time. Speed or direction changes.
    • Scalar Quantities: Defined by magnitude only (e.g., Speed, Mass, Time).
    Quantity Nature Definition
    Speed Scalar Distance / Time
    Velocity Vector Displacement / Time
    Weight Vector Force due to gravity
    Average Speed Formula:

    $Average\ Speed = \frac{Total\ Distance}{Total\ Time}$

    SI Unit: m/s

    ❌ Common Error:

    Assuming an object moving in a circle at constant speed is in uniform motion. Fix: It is Non-Uniform because its direction (velocity) is constantly changing.

    Distance-Time Graph (Uniform Motion)

    ⚡ Fast Revision: Weight & Mass

    Mass vs. Weight
    • Mass: The quantity of matter contained in a body. It remains constant everywhere in the universe.
    • Weight: The force with which the Earth attracts an object towards its center.
    • Variation: Weight changes from place to place (e.g., your weight on the Moon is $1/6^{th}$ of your weight on Earth).
    Feature Mass Weight
    Instrument Beam Balance Spring Balance
    SI Unit Kilogram (kg) Newton (N)
    Quantity Type Scalar Vector (Downwards)
    The Weight Formula:

    $W = m \times g$

    where $g \approx 9.8\ m/s^2$ (Acceleration due to gravity)

    ❌ Common Error:

    Using "kg" for weight in physics problems. Fix: In physics, mass is measured in **kg**, but weight must always be expressed in **Newtons (N)** or **kgf**.

    Measuring Mass vs. Weight

    ⚡ Fast Revision: Numerical Motion Relationships

    Speed & Time Graphs
    • Slope of d-t Graph: Represents the speed of the object. A steeper slope means higher speed.
    • Horizontal Line (d-t Graph): Represents an object at rest (speed = 0).
    • Curved Line (d-t Graph): Represents non-uniform motion (acceleration or deceleration).
    Unit Conversion Shortcut:

    To convert km/h to m/s: Multiply by $\frac{5}{18}$

    To convert m/s to km/h: Multiply by $\frac{18}{5}$

    Standard Values

    Speed of Sound: $\approx 330$ to $340$ m/s

    Speed of Light: $3 \times 10^8$ m/s

    Graph Feature Interpretation (Uniform Motion)
    Straight line through origin Constant Speed
    Straight line parallel to x-axis Stationary (Zero Speed)
    Variable slope Changing Speed
    ❌ Common Error:

    Adding speeds directly to find average speed (e.g., $(v_1+v_2)/2$). Fix: You must always use Total Distance / Total Time for accurate results.

    End of Motion Revision
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