Try to move a loose rock. Try to catch a cricket ball. Both will be difficult. Starting a body at rest or stopping a moving body requires force. A body always resist the change. The resistance level is proportional to the mass of the body. This property of the body is called 'Inertia'. So we can say mass is the measure of inertia or vice versa.
In earth, gravitational force acts on all the bodies. We measure it and call it weight. In space, in the absence of gravity, pure mass and inertia can be measured using artificial force.
Once we overcome the 'inertia' (resistance) and start the body, it is easy to keep it going. That is why, starting of the vehicles and machines requires lot of energy. The satellites are pushed( by rocket) once into the orbit and it keeps on resolving around the earth for number of years.
In rotational motions, inertia is not enough. We have to calculate the "moment of inertia". It means the rotating mass should be multiplied by the square of the distance between itself(mass) and the axis of rotation.
In the wheels of the all the vehicles, more mass (Tyre, air) is present on the outer edge-rim. Hence distance from the axis of rotation increases and the moment of inertia increases. So the wheels in motion cannot be easily disturbed and this will aid the saving of the fuel. The inertia-laziness is also useful.
We, humans also have inertia. It is difficult to start any work. Once started, it will move on. So initiate and start the most difficult work tomorrow morning. It is from human psychology.
Note: When you travel in a speeding car and the car is suddenly stopped by braking; your body will not be stopped due to inertia. Hence, you will dash against the dashboard. To avoid this, wear seat belt ; attach yourself with the car and be a part of the car.
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