Hello Simra Shah,
I would like to tell you that Inertia is the resistance of the object to any change in its motion, including a change in direction. An object will stay still or keep moving at the same speed and in a straight line unless it is acted upon by an external unbalanced force.
For more detailed info about inertia, I would suggest you visit the below wiki thread.
The link for the same is: https://simple.wikipedia.org/wiki/Inertia
Hope this helps and all the very best!
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