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Force

Force is an external agency which changes or tends to change the state of rest or uniform motion along a straight line. It is a vector quantity and its SI unit is newton.
 

 

Newton’s First Law Of Motion
 
Consider a ball rolling at some speed on a horizontal floor. We find that its speed decreases and it stops after travelling some distance. The floor exerts a force of friction on the ball, and it is this force that is responsible for slowing down the speed of the ball. If the floor is made smoother, the force of friction reduces and the ball moves through a longer distance. Imagine a situation when we get rid of friction completely. Then the ball will continue to move without slowing down or changing direction forever or until it hits an obstacle.
Thus, we have seen that
  1. If a body is at rest and no force acts on it, it remains at rest.
  2. If the body is moving along a straight line and no force acts on it, it continues to move on the same line with a constant velocity.
  3. If a force acts on a body, the body will accelerate or change direction.
These facts taken together form the Newton’s first law of motion, stated as follows:
 
Every body continues to be in a state of rest or uniform motion along a straight line unless an external force acts upon it.

Inertia

The fundamental property of a body to continue in its state of rest or of uniform motion is called inertia. In addition, the inability of a body to change its state of rest or of uniform motion on its own is called inertia.

Since the first law is about inertia, it is known as the law of inertia.

We can study two forms of inertia, namely
  1. inertia of rest and
  2. inertia of motion.
Inertia of rest A body at rest has the tendency to continue in its state of rest unless an external force is applied to change the state of rest. This tendency is called inertia of rest.
 
Example

When a bus suddenly moves, a person standing in it tends to fall backward. This is because, the lower part of his body (his feet) in contact with the bus begins to move with the bus, but the upper part of his body, due to inertia, would tend to remain at rest. This causes the person to fall backward. This is due to inertia of rest.

 

Inertia of motion
 A body in a state of uniform motion along a straight line possesses inertia of motion, i.e., it has a tendency to continue in the state of uniform motion along a straight line, unless an external force is applied to change its state.
 
Example

A person riding a bicycle along a level road does not come to rest immediately after he stops peddling. The cycle continues to move forward due to the inertia of motion. But finally it comes to rest, after travelling some distance as a result of the retarding force of friction.

 

When a bus suddenly stops, a person falls forward. This is due to inertia of motion.

 

 

Inertia and Mass

Mass of an object is defined as the quantity of matter contained in it.

Consider two bodies of unequal masses initially at rest. If we push the bodies equally hard for equal time, both will start moving. But, the lighter one acquires a larger velocity and the heavier one gets a smaller velocity (Figure 3.1).
 


 

Thus, the heavier one has resisted the change more effectively than the lighter one. Hence, heavier bodies have larger inertia than lighter ones. So, mass is the quantitative measure of inertia.





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