9th Class Science Force and laws of motion Inertia


Category : 9th Class

*      Inertia


Inertia is the property of a body due to which the body opposes any change in its state of rest or of uniform motion along a straight line. Inertia is the measure of mass. More mass means more inertia.


*              Inertia of Rest

A body which is at rest will remain at rest unless or until some external force is applied to it.

It is the inability of a body to move on its own.


*           Activity

  • If we give sharp horizontal flick to the card then the card shoots away and the coin falls into the glass.
  • This happens due to inertia of rest of coin. The coin tends to remain in its state of rest and hence falls into the glass.



*            Inertia of Motion

  • It is the inability of a moving body to stop on its own.




Rahul is being chased through the woods by a bull moose which he was attempting to photograph. The enormous mass of the Bull Moose is extremely intimidating. Yet, if Rahul makes a zigzag pattern through the woods he will be able to use the large mass of the moose to his own advantage. Explain this in terms of inertia and Newton's first law of motion.


The large mass of the Bull Moose means that the Bull Moose has a large inertia. Thus, Rahul can more easily change his own state of motion make quick changes in direction while the moose has extreme difficulty changing its state of motion.


*           Inertia of Direction

The tendency of a body by which it resists a change in its direction of motion is called inertia of direction.




       Yatin spends most Sunday afternoons at rest on the sofa, watching football games and consuming large quantities of food. What effect (if any) does this practice have upon his inertia?

(a) Increases

(b) Decreases

(c) No effect

(d) All of these

(e) None of these

Answer: (a)



        Small droplets of water fall down when you shake a wet piece of cloth due to _____.

(a) Inertia of rest of droplets

(b) Inertia of motion of droplets

(c) Inertia of direction of droplets

(d) None of these

(e) All of these


Answer: (a)

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