Inertia

Why does a person sitting in a stationary vehicle get pulled back when the vehicle starts moving?

In order to understand this phenomenon you perform an experiment. Cover the mouth of a glass tumbler with a piece of cardboard and place a coin on it. Now push the cardboard with your finger so that the cardboard is thrown off the tumbler. Do you know what would happen? Along with the cardboard the coin would also fall off the glass. You should not worry and once again place the cardboard atop the glass along with a coin. Now flick the cardboard forcefully with your forefinger propelled with the help of your thumb and observe the consequences.

This time it would be quite different from what happened earlier. As soon as you strike the cardboard it would be thrown off but the coin would drop in the glass tumbler. What actually happens is this; the cardboard is thrown off with such a force that the coin fails to accompany the cardboard and the cause of the failure of the coin to move together with the cardboard is law of inertia. When a thing is stationary it remains in that stage till an external force makes it move. This is called Law of inertia.

Inertia and Mass

Newton's first law of motion states that "An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force." Objects tend to "keep on doing what they're doing." In fact, it is the natural tendency of objects to resist changes in their state of motion. This tendency to resist changes in their state of motion is described as inertia.

Inertia: the resistance an object has to a change in its state of motion.

                Newton's conception of inertia stood in direct opposition to more popular conceptions about motion. The dominant thought prior to Newton's day was that it was the natural tendency of objects to come to a rest position. Moving objects, so it was believed, would eventually stop moving; a force was necessary to keep an object moving. But if left to itself, a moving object would eventually come to rest and an object at rest would stay at rest; thus, the idea that dominated people's thinking for nearly 2000 years prior to Newton was that it was the natural tendency of all objects to assume a rest position.

 Mass as a Measure of the Amount of Inertia

All objects resist changes in their state of motion. All objects have this tendency - they have inertia. But do some objects have more of a tendency to resist changes than others? Absolutely yes! The tendency of an object to resist changes in its state of motion varies with mass. Mass is that quantity that is solely dependent upon the inertia of an object. The more inertia that an object has, the more mass it has. A more massive object has a greater tendency to resist changes in its state of motion.

Suppose that there are two seemingly identical bricks at rest on the physics lecture table. Yet one brick consists of mortar and the other brick consists of Styrofoam. Without lifting the bricks, how could you tell which brick was the Styrofoam brick? You could give the bricks an identical push in an effort to change their state of motion. The brick that offers the least resistance is the brick with the least inertia - and therefore the brick with the least mass (i.e., the Styrofoam brick).

A common physics demonstration relies on this principle that the more massive the objects, the more that object resist changes in its state of motion. The demonstration goes as follows: several massive books are placed upon a teacher's head. A wooden board is placed on top of the books and a hammer is used to drive a nail into the board. Due to the large mass of the books, the force of the hammer is sufficiently resisted (inertia). This is demonstrated by the fact that the teacher does not feel the hammer blow.

BY

KISHORE, NSS VOLUNTEER