It is clear from Galileo’s experiments that all objects have a tendency to continue in their state of rest or of uniform motion until an external force acts on it. The following examples will help to understand the observations of Galileo’s experiment.
Cardboard and a Coin placed on an Empty Tumbler
Flick the cardboard with the finger. What do you observe? The coin drops into the tumbler. When we flick the cardboard the cardboard moves fast whereas the coin continues in its state of rest and hence drops into the tumbler.
Coin Drops into the Tumbler as the Cardboard is Flicked
A passenger standing in a moving bus leans forward when the brakes are applied all of a sudden. This is because the body of the passenger is in motion along with the bus. When the bus stops all of a sudden, the lower part of his body comes to rest along with the bus whereas the upper part of the body continues to move forward.
From the above examples it is clear that objects continue to remain in their state of rest or of uniform motion until an external force is applied. This tendency of an object to resist any change in its state of rest or of uniform motion is called inertia.