Newton's First Law of Motion: An object at rest or in uniform motion will remain at rest or in uniform motion unless an unbalanced external force acts on it.
According to this law an object will no change its state of motion and it will not change its velocity unless and until an external force is applied to it.
For eg. A car which is at rest will not move unless the engine applies force.
The same car with velocity v will continue moving with the same velocity unless an external force is applied to it.
The same car will not stop unless an external force (like friction and brakes) is applied to it such that it comes to rest.
Newton's Second Law Of Motion: The time rate of change in momentum is directly proportional to the resultant of external force and the change in momentum is in the direction of the resultant external force.
According to this law, if momentum is denoted by p and time by t, then
Δp/t is proportional to F.
This means that if momentum is more then the moving body applies more force and if it is less then the force applied by the moving body is less.
Similarly, if the time period during which the force acts is more then the force applied will be less. If the time period during which the force acts is less then the force applied will be more.
For eg, When a cricketer is about to receive a catch, the ball is coming towards him with greater velocity and has more momentum. When he receives the catch, he pulls his hands backwards. In this way he increases the time period during which the force acts and reduces the force applied. So he doesn't get hurt.
Newton's Third Law Of Motion: During interaction between two bodies, the force exerted by the first body on the second body is equal and opposite to the force exerted by the second body on the first body.
This means that forces always exist in pairs.
For eg. When we are walking, we apply force on the ground which is called action. At the same time, the ground applies equal and opposite force on us which is called reaction. Due to this, we are able to walk on the earth.