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Motion, Class 9 Science

By  May 25, 2021  

Motion - Class 9 Science

Motion - Motion is the movement of any object from one point to another with respect to observer. An object is said to be in motion when it changes its position with time.

Rest - A body is said to be at rest if it does not change its position with respect to its surroundings with time.

Different Types of Motion

  • Linear Motion - Linear motion is a motion in which a body moves in a straight line. Example - Marchpast by soldiers, A car moving on a straight road.
  • Circular Motion - Circular motion is a motion in which a body moves along in a curved path. Example - Motion of satellites around planets, A car moving in a traffic roundabout
  • Rotational Motion - Rotational motion is a motion in which a body rotates about a fixed axis. Example - Motion of wheels around vehicles, Spinning of a top
  • Vibratory Motion - Vibratory motion is a motion in which a body moves in a to and fro direction. Example - Motion of a simple pendulum, Motion described by the string of a violin when

    • Rest and Motion are relative terms - Rest and motion are considered as relative terms because they both depend on observer's frame of reference.

    Uniform motion - When an object covers equal distance in equal intervals of time.

    Non-uniform motion - When an object covers unequal distance in equal intervals of time.

    Circular motion - Circular motion is the movement of an object along the circumference of a circle or rotation along a circular path.

    Uniform circular motion - Uniform circular motion can be described as the motion of an object in a circle at a constant speed.

    Non-uniform circular motion - Non-uniform circular motion denotes a change in the speed of a particle moving along a circular path.

    Direction of motion at any point in a circular path - As an object moves in a circle, it is constantly changing its direction. At all instances, the object is moving tangent to the circle.

    Equation of speed in a uniform circular motion, if the radius of the circle is r and time taken is t: v = circumference/time or v = 2πr/t, Note - Here v is scalar and not vector.

    Circumference of Circle = 2πr (r is the radius of the circle)
    Difference between Circular motion and linear motion
    Circular motion Linear motion
    In circular motion, the speed is constant but the direction of the object changes continuously, hence it is accelerated In linear motion, the speed and direction of the object is fixed and so it is not accelerated
    Motion of earth around the sun A car moving on a straight road

    Magnitude - Magnitude is the size or extent of a physical quantity.

  • Scalar quantity - Scalar quantity are only expressed only in magnitude. Example - Time, Distance, Mass, Temperature, Length, Area, Volume etc.
  • Vector quantity - Vector quantity are expressed in magnitude as well as the direction of the object. Eg. Velocity, Displacement, Weight, Force, Acceleration etc.
    • Difference between scalar and vector quantity
    Scalar Quantity Vector Quantity
    They are expressed in magnitude only They are expressed in magnitude and direction
    They can be added by simple arithmetic means They cannot be added by simple arithmetic means
    They cannot be easily plotted on graph They can be plotted on graph
    They are one dimensional They are multidimensional
    Area, Pressure, Density, Temperature, Mass Momentum, Force, Acceleration, Displacement, Velocity

    Distance - It is the actual length of the path travelled by a moving body irrespective of the direction in which the body moves.

    Displacement - The shortest distance of a moving body from the point of reference(initial position of body).

    Difference between Distance and Displacement
    Distance Displacement
    It is the actual length of path travelled by a moving body It is the shortest distance between the initial and final position of the body
    It has only magnitude(scalar quantity) It has both magnitude and direction(vector quantity)
    It is always positive It may be positive, negative or zero
    SI unit of distance is m SI unit of displacement is m

    Speed - Speed is the defined as the distance covered in unit time. Speed = Distance/Time

    Average Speed - Average speed is the total distance travelled in a particular interval of time or we can say average speed is the total distance travelled in total time interval. Average speed = Total Distance/Total time

    Uniform speed - When the distance travelled by a body is equal in equal interval of time.

    Non-uniform speed - When the distance travelled by a body is unequal in an equal interval of time.

    Velocity - Velocity is the displacement of the object in unit time. Velocity is speed with direction. Example - Car moving at 40km/h is its speed whereas the car moving at 40km/h westward is the velocity

    Difference between speed and velocity
    Speed Velocity
    Speed is the distance covered in unit time. Speed = Distance/Time Velocity is the displacement covered in unit time. Velocity = Displacement/Time
    Speed is a scalar quantity Velocity is a vector quantity
    Speed is rate of change of distance Velocity is rate of change of displacement
    Speed can never be zero Velocity can be positive, negative, zero
    SI unit of speed is m/s SI unit of velocity is m/s

    Acceleration(a) - Rate of change of velocity, a = change of velocity/time or, a = (v - u)/t, here v - final velocity, u - initial velocitySI unit of acceleration is m/s2

    Uniform acceleration - Uniform acceleration is the acceleration in which the velocity of a body changes at a constant rate in a given interval of time.Example - Motion of a ball dropped from a height.

    Non-uniform acceleration - Non-uniform acceleration is the acceleration in which the the velocity of a body does not change at a constant rate in a given interval of time.Example - A car travelling 60 km in 1 hour and 70 km in 2nd hour

    Equations of motion

  • v = u + at
  • s = ut + 1/2 at2
  • v2 = u2 + 2as

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