Newton's Laws of Motion

First Law of Motion: Everybody in the Universe continues in its own state of rest or of uniform motion along a straight line in the absence of an external force.

For example, a book lying on a table remains there unless a net external force acts on it.  A rolling ball on a flatland would  remain rolling forever if there were no friction between the ball and the ground. 

Newton's First Law of motion is also called Galileo's Law of Inertia. Why?  

An object on itself is inert. This means that a moving object tends to be in the state of motion and an object at rest tends to be at rest if no external force acts upon them. This property is called inertia of the object. The tendency of a moving object to be in the state of motion is called the inertia of motion. For example, passengers in a moving vehicle are thrown forward when the vehicle suddenly stops. It is because the lower part of the body of a passenger which is in contact with the vehicle comes to rest when the vehicle suddenly stops but the upper part of the body of the passenger tends to continue in the state of motion due to inertia of motion. Therefore, the passenger falls forward. On the other hand, the passengers are thrown backwards when a vehicle suddenly starts. The lower part of the body of passenger which is in contact with the vehicle suddenly comes to motion but the upper part tends to be in the state of rest due to inertia of rest. Therefore, the passenger falls backward.

Second Law of Motion: The rate of change of momentum of an object is directly proportional to the net force acting on the object, and it takes place in the direction of the force. 

Momentum of an object measures its total quantity of motion. Mathematically, it is the product of the mass of an object and its velocity. 

If an object is under the influence of net force, its velocity keeps on changing continuously. Therefore, the momentum of the object, which is equal to the product of its mass and velocity also changes with time. For a given time interval, the change in momentum of the object divided by the time interval is higher if the force is stronger and this change takes place in the direction of the net force acting on the object. 

Third Law of Motion: To every action, there is an equal and opposite reaction.

If an object exerts a force on an another object, the second object exerts exactly the same amount of force on the first object in the opposite direction. A very important point to note is that the action-reaction pair of forces always act on different objects, and they, in general do not cancel each other ! 

Summary:

(1) All the three laws of motion describe the same physical quantity - Force. 

(2) The First Law of Motion defines force, the Second Law of Motion measures force and the Third Law of Motion gives the property of force. 

Test Yourself:

(1)  A horse rider falls backward when the horse suddenly starts but falls forward when the horse suddenly stops. Why?

(2) An athlete runs through some distance before making a long jump. Why?

(3) A gun recoils while shooting. Why? 

(4) A roll of tissue unwinds without snapping if  the sheet of tissue is pulled gently but snaps into pieces if it is jerked. Why?

(5) It is difficult to walk in ice or sand. Why?

(6) Fruits fall when the branches are shaken. Why?

(7) If an object of one kilogram moving with a velocity of  one meter per second changes its velocity to two meters per second in two seconds, what is the net force acting on the object?

(8) There are a number of thick hardcover books nicely stacked on top of each other. You are asked to pull out a book from the middle of the stack without moving out other books. How do you do that?

(9) Put a postcard on top of a glass and a penny on the middle of the postcard. What happens to the coin if you pull the card horizontally (a) slowly (b) rapidly? Why?

(10) Place carefully a post card on top of a stick which is standing on a vessel filled with sand. If you hit the stick on its middle horizontally, the stick will fall out of the vessel, but the post card falls on the vessel itself. Why?
(11) "Newton's First Law of motion can be obtained from the Newton's Second Law of motion in a special case." Explain.