CBSE – 9th Standard Science

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Class 9 Chapter 8 – Motion

  • Movement of any object from one place to other in a uniform or non uniform speed is called Motion
  • To describe the position of an object we need to specify a reference point called the origin.
  • The simplest type of motion is the motion along a straight line.
  • The shortest distance measured from the initial to the final position of an object is known as the displacement.
  • As the object covers equal distances in equal intervals of time, it is said to be in uniform motion.
  • As the object covers unequal distances in equal intervals of time, it is said to be in non uniform motion.

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Graphical representation of motion

  1. Distance -time graph

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SPEED

  • One of the ways of measuring the rate of motion of an object is to find out the distance travelled by the object in unit time. This quantity is referred to as speed.
  • The SI unit of speed is meter per second represented by the symbol m s-1 or m/s.
  • The other units of speed include centimeter per second (cm s-1) and kilometer per hour (km h-1).
  • To specify the speed of an object, we require only its magnitude.

AVERAGE SPEED

  • The average speed of an object is obtained by dividing the total distance travelled by the total time taken.

VELOCITY

  • The quantity that specifies both the speed and direction is called velocity.
  • Velocity is the speed of an object moving in a definite direction.
  • The velocity of an object can be uniform or variable.
  • In case the velocity of the object is changing at a uniform rate, then average velocity is given by the arithmetic mean of initial velocity and final velocity for a given period of time.

Velocity-time graph

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ACCELERATION

  • Acceleration is a measure of the change in the velocity of an object per unit time.
  • If the velocity of an object changes from an initial value ‘u’ to the final value ‘v’ in time ‘t’, the acceleration ‘a’ is, a = (v-u)/t.
  • This kind of motion is known as accelerated motion.
  • The acceleration is taken to be positive if it is in the direction of velocity.
  • The acceleration is taken to be negative when it is opposite to the direction of velocity.
  • The SI unit of acceleration is m s-2.
  • If an object travels in a straight line and its velocity increases or decreases by equal amounts in equal intervals of time, then the acceleration of the object is said to be uniform motion.
  • If an object can travel with non-uniform acceleration and if its velocity changes at a non-uniform rate then the acceleration is said to be Non uniform motion.

GRAPH OF MOTION

  • To describe the motion of an object, we can use line graphs.
  • In this case, line graphs show dependence of one physical quantity, such as distance or velocity, on another quantity, such as time.
  • For uniform speed, a graph of distance travelled against time is a straight line.

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EQUATIONS OF MOTION

  • When an object moves along a straight line with uniform acceleration, it is possible to relate its velocity, acceleration during motion and the distance covered by it in a certain time interval by a set of equations known as the equations of motion.
  • There are three such equations.
  • Velocity Time Relation Equation v = u + at.
  • Position Time Relation Equation s = ut + 1/2 at2.
  • Position Velocity Relation Equation 2as = v2 – u2.
  • Where u is the initial velocity of the object which moves with uniform acceleration a for time t, v is the final velocity, and s is the distance travelled by the object in time t.

UNIFORM CIRCULAR MOTION

  • When an object moves in a circular path with uniform speed, its motion is called uniform circular motion.

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