Newton's Laws of Motion - Part III

4. Newton’s second law of motion 

From our earlier discussions, Newton’s second law of motion neatly follows.  

Newton’s second law of motion states that the rate of change of momentum of a body is directly proportional to the force acting on it and takes place in the direction of the force. 

To elaborate the statement of the law, we can say that

• A force changes the velocity of a body. That is the body is accelerated.

• A force changes the momentum of the body. The momentum changes in the direction in which the force is acting.

• Once the force is removed, the acceleration becomes zero. The body returns to its state of rest or uniform motion. 

5. Newton’s third law of motion 

Newton’s first and second laws of motion deal with force as a cause, and change of motion as its effect. The third law tells about mutual action between two bodies. 

To understand what is mutual action-interaction between two bodies : do the following. Sit with a chair near a wall. Press your feet against the wall. You will see that instead of the wall moving, your chair is moving backwards!! The harder you push, more inclined your chair becomes. From the Newton’s first and second law you know that the cause for your chair’s motion must be a force.  Now where is the force coming from? Just as you are pressing the wall, the wall in turn is pressing you! Because the wall has a larger mass, it doesn’t move. But you do!!

The same type of action-interaction you observe when a rocket is fired. The fuel is emitted with force in one direction and the rocket flies off in the opposite direction.    For every force produced on one body there is a reaction force. It is of the same magnitude and acts in the opposite direction. It acts on the body which produced the first force. Out of the two forces involved, one is called the action force and the other is called the reaction force.    Newton’s third law states that to every action there is always an equal and opposite reaction.    It should be borne in mind here that the action and reaction forces act on different bodies, and although they are equal and opposite, their resultant is not zero.

6. Law of conservation of momentum 

When two bodies collide, their velocities will not the remain same as that before collision. In absence of any other external force, the momentum before and after collision of the bodies, together should remain invariant. Example of this is seen in our everyday lives : when we fire a bullet from a gun, the gun suffers a recoil, when we play carom, we see that the striker transfers its momentum to a coin etc. 

Law of conservation of momentum states that, the total momentum after impact is the same as the total momentum before impact. 

The law of conservation of momentum is one of the most basic laws in physics.  We see it coming into play while studying particle creation and annihilation.

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