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Newton's Laws of Motion - Part II |
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2. Concept of Inertia We have seen that a body
preserves its state of rest or motion as long as no net force is acting on
it. A body has a tendency not to change its state; it is reluctant or
inactive. The tendency of a body to maintain its state (of rest or motion)
is known as inertia. Larger the
inertia of a body, more difficult it is to change its state of rest or
uniform motion. We have seen this in our above example of a marble and a
book. The book has larger inertia than the marble. A very simple experiment shows the idea of inertia very clearly. Take
an empty glass and place a post card to cover the glass. Put a fifty paisa coin on the post card. Now flick the card off the glass. The
coin, instead of moving with the post card, falls into the glass. This
demonstrates that the coin is reluctant to change its state of motion along
with the post card. It displays inertia.
While
traveling in a car, you will notice when the car starts, you
feel a force pressing you against the seats, in opposite direction to the
motion of the car. This is again the force of inertia. At the end of the
journey, when the car stops, the passengers lean forward, as if they want to
continue moving forward! This
is another case of passenger's showing inertia or resistance to their
changed state of motion. A logical conclusion of the above discussion is that mass is a
measure of the inertia of a body. Higher the mass, greater would be its
inertia, as we have seen in the example of a marble and a book. 3. Concept
of momentum
If a ping pong ball and a football are rolling towards you at the same speed, then you will notice that to bring them to a halt, you will have to apply larger force for the football. The ping pong ball stops easily. In another situation, if two footballs are moving towards you at different speed, then you will notice that the football with the higher speed is more difficult to stop. It appears from these observations that the force applied to stop depends on both the mass and the velocity of the body. The quantity of motion in a body depends on the mass and its velocity. The new term is called momentum. Momentum is the product of mass and velocity of the moving body. It is a vector quantity and is written as
If we go back to the definition of force :
Acceleration is rate of change of velocity
time duration for the change From
combining all the three above equations, we can say that force is rate of
change of momentum.
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