Class 9 Physics Motion Acceleration


  • Acceleration: It is a measure of the change in the velocity of an object per unit time.
  • Acceleration can be caused either by change in direction of motion or change in speed or both. 

  • SI unit is ms-2.
  • It is a vector quantity; the acceleration is taken to be positive if it is in the direction of velocity and negative when it is opposite to the direction of velocity.


  • When you apply brakes on a car the velocity is in forward direction but acceleration is in backward direction, so acceleration will be negative. It is also called deceleration.


  • When you are increasing the speed of a car the acceleration is positive as velocity and acceleration are in both in the forward direction



(The acceleration in this jet pack is provided by thrust)

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