Class 9 Physics Chapter 1: Motion – Questions and Answers (Theoretical)

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🚀 Class 9 Physics Chapter 1: Motion – Questions and Answers (Theory Only)

These questions are based on CBSE NCERT Class 9 Physics Chapter "Motion". These are fully theory-based and are designed for concept-building and exam preparation. No numerical problems included.

📘 1. Describing Motion

  1. What do you mean by the term "motion"?
    → Motion is the change in position of an object with respect to time and a reference point.
  2. Can an object be in motion for one observer and at rest for another? Explain.
    → Yes. Motion is relative. A person sitting in a train is at rest w.r.t the train but in motion w.r.t the ground.
  3. How can we say that motion is a continuous process?
    → Because even when an object appears to be at rest, its molecules are still in motion.
  4. Is motion always visible?
    → No. Motions like sound waves or atoms moving are invisible but real.
  5. Why is reference point important to describe motion?
    → Because motion is always measured relative to some fixed point.

📘 2. Motion Along a Straight Line

  1. What is rectilinear motion?
    → Motion in a straight line. E.g., a train moving on a straight track.
  2. What is numerical ratio of displacement to distance for a moving object?
    → Always less than or equal to 1. (Displacement is always less than or equal to the distance)

    3. 📘 Difference and Similarities between Distance and Displacement

    Point Distance Displacement
    1. Definition Total path length covered by a moving object. Shortest path between the initial and final position of the object.
    2. Type of Quantity Scalar quantity (has magnitude only). Vector quantity (has both magnitude and direction).
    3. Direction Does not consider direction. Considers direction.
    4. Value Always positive or zero. Can be positive, negative, or zero.
    5. Comparison Equal to or more than displacement. Equal to or less than distance.
    6. Path Dependence Depends on the actual path taken. Depends only on initial and final positions.
    7. SI Unit Metre (m) Metre (m)
    8. Example If a car moves in a circle and returns to starting point, distance = full circle. If it returns to start, displacement = 0.

📘 3. Uniform and Non-Uniform Motion

  1. Define uniform motion with example.
    → An object is said to be in uniform motion if it covers equal distances in equal intervals of time, in a straight line.
    📌 This means:
    • Speed is constant
    • Direction is constant
    • Velocity is also constant
    • Acceleration is zero

    ✅ Example: A car moving at 40 km/h in a straight line without changing direction.
  2. The displacement of an object is proportional to square of time, then the object moves with:
    1. uniform velocity
    2. uniform acceleration
    3. increasing acceleration
    4. decreasing acceleration

    →

    ✅ Answer:

    The object moves with uniform acceleration.

    🧠 Explanation:

    When displacement (s) is proportional to the square of time (t²), we can write:

    s ∝ t²

    This kind of relationship appears in the equation of motion:

    s = ut + (1/2)at²

    If the object starts from rest (i.e. initial velocity u = 0), then the equation becomes:

    s = (1/2)at²

    This shows that displacement is directly proportional to t², which is a clear sign of motion under uniform (constant) acceleration.

    🔍 Conclusion:

    If s ∝ t², then the object is undergoing uniform acceleration.

  3. Which type of motion is common in real life?
    → Non-uniform motion due to traffic, obstacles, etc.

  4. What is Non-uniform Motion?

    Answer:

    An object is in non-uniform motion if it covers unequal distances in equal intervals of time, or if its direction keeps changing.

    📌 This means:

    • Speed or direction (or both) are changing
    • Velocity is not constant

    ✅ Example:

    A car slowing down at a red light, or a bus turning at a curve.

  5. Is Uniform Motion the same as Uniform Speed?

    Answer:

    No, they are not the same.

    Uniform speed means only speed is constant, while direction may change.
    Uniform motion means both speed and direction are constant — so velocity is constant.

    📊 Difference between Uniform Speed and Uniform Motion:

    Term Meaning
    Uniform Speed Only speed is constant, direction may change.
    Uniform Motion Speed and direction both are constant → so velocity is constant.

📘 4. Speed with Direction (Velocity)

  1. What is velocity?
    → Speed in a given direction.
  2. Can velocity be negative?
    → Yes, depending on direction of motion.

  3. 📊 Comparison between Speed and Velocity

    Point Speed Velocity
    1. Definition Rate of change of distance with time. Rate of change of displacement with time.
    2. Quantity Type Scalar quantity (has only magnitude). Vector quantity (has magnitude and direction).
    3. Direction Does not consider direction. Considers direction.
    4. Value Always positive or zero. Can be positive, negative, or zero.
    5. Formula Speed = Distance / Time Velocity = Displacement / Time
    6. SI Unit Metre per second (m/s) Metre per second (m/s)
    7. Example A car moving at 60 km/h without considering direction. A car moving 60 km/h towards east (direction matters).

📘 5. Rate of Change of Velocity (Acceleration)

  1. What is acceleration?
    → It is the rate of change of velocity. It is a vector quantity.
  2. Can acceleration be zero?
    → Yes, if velocity is constant.
  3. Can acceleration be negetive?
    → Yes, It is a vector quantity. Negetive acceleration means object is moving in negetive direction or it is redardation.
  4. What is retardation?
    → The decrease in the velocity of a body over time is known as retardation. It is also known as negetive acceleration. Example, When brakes are applied to a vehicle, causing its velocity to decrease.
  5. A boy is enjoying a ride on a merry-go-round which is moving with a constant speed of 10 m/s. What type of motion is this?
    → It is accelerated motion . Even though the speed is constant, the direction is continuously changing because the motion is circular. so its velocity is changing.
    velocity = speed + direction
    Changing direction = changing velocity = acceleration
    Note:- This motion can also be called Uniform Circular Motion as the speed is constant and the path is circular.
  6. Can an object have zero velocity and still have acceleration?
    → Yes. At the highest point of a ball thrown upward, its velocity is zero, but acceleration is still acting downwards due to the gravity.
  7. What is uniform acceleration?
    → If the velocity of an object increases or decreases by equal amounts in equal time intervals, it is said to have uniform acceleration.

    Example: A freely falling ball under gravity accelerates uniformly at 9.8 m/s².
    Key Point: In uniform acceleration, the value of acceleration remains constant.
  8. What is non-uniform acceleration?
    → If the velocity of an object changes by unequal amounts in equal time intervals, it has non-uniform acceleration.

    Example: A car moving through traffic speeds up and slows down unpredictably.
    Key Point: In non-uniform acceleration, the acceleration is not constant.

📘 6. Distance–Time Graphs

  1. What does a straight line mean in a distance-time graph?
    → Uniform motion.
  2. What does a curved graph indicate?
    → Non-uniform motion.
  3. What does a straight line parallel to the time axis (X-axis) mean in a distance-time graph?
    or
    What does a horizontal line in a distance-time graph mean?
    → Object is in rest.
  4. What does a straight line parallel to the distance axis (Y-axis) mean in a distance-time graph?
    or
    What does a vertical line in a distance-time graph mean?
    → It is not possible in real life. because distance is changing but time is constant.Thus this graph is not possible.
  5. What does a slope in a distance-time graph represent?
    → A Slope in a distance-time graph represents Speed
    Steeper Slope → faster speed
    Gentle Slope → slower speed
    Zero Slope (Horizontal line) → Rest

📘 7. Velocity–Time Graphs

  1. What does the slope of a velocity-time graph give?
    → The slope represents Acceleration.
    Steeper Slope → more acceleration
  2. What does the area under the velocity-time graph represent?
    → Displacement.
  3. What does a straight horizontal line in a velocity–time graph mean?
    → It means the object is moving with constant velocity. Acceleration is zero.
  4. What does a sloped straight line mean in a velocity–time graph?
    → It means the object has uniform acceleration.
    Upward slope → positive acceleration
    Downward slope → negative acceleration (retardation)
  5. When is displacement positive?
    → If the velocity–time graph lies above the time (x) axis, the velocity is positive.
    → The object is moving in the forward direction, so the displacement is positive.
  6. When is displacement negative?
    → If the velocity–time graph lies below the time axis, the velocity is negative.
    → This means the object is moving in the opposite direction (backward), so the displacement is negative.
  7. What if the graph has parts above and below the time axis?
    → Displacement is calculated by adding positive and negative areas (algebraic sum).
    → Final displacement depends on which area is bigger.

📘 8. Equations of Motion

  1. State the first equation of motion.
    → v = u + at
  2. Why are equations of motion only valid for uniform acceleration?
    → Because they are derived assuming constant 'a'.
  3. What kind of motion do these equations describe?
    → They describe motion in a **straight line** with **constant acceleration** only. They are not valid for rotational or irregular motion.
  4. Are equations of motion applicable for all types of motion?
    → No. These equations are valid only for motion with uniform accelerationalong a straight path, not along a curve or in a circle. .
  5. Can we use these equations if the object has non-uniform acceleration?
    → No. If acceleration is non-uniform (changing at different rates), then the equations of motion do not give accurate results.
  6. Are equations of motion valid for circular motion?
    → Not directly. In **circular motion**, even if speed is constant, the direction of velocity changes, which means velocity is not uniform. Hence, these equations cannot be used as they assume straight-line (linear) motion.
  7. Can equations of motion be used at very high speeds close to the speed of light?
    → No. These equations are based on **classical mechanics** and fail at very high speeds (relativistic speeds), where **Einstein’s theory of relativity** is required.

📘 9. Uniform Circular Motion

  1. Define Uniform Circular Motion?
    → Uniform Circular Motion is the motion of an object moving in a circular path at a constant speed.
  2. Why is circular motion considered accelerated?
    → Because direction of velocity changes constantly.
  3. Name the force that keeps object in circular motion.
    → Centripetal force.

🧠 Tip: Read these questions again and try explaining each answer in your own words. It will improve your understanding and confidence in theory-based questions.

More chapters coming soon. Stay connected with Horizon Academy!

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