The first law refers to the simple case when the net external force on a body is zero. The second law of motion refers to the general situation when there is a net external force acting on the body. It relates the net external force to the acceleration of the body.
Momentum -
Momentum, P of a body is defined to be the product of its mass m and velocity v, and is
denoted by p: p = m v
Momentum is clearly a vector quantity. The following common experiences indicate the
importance of this quantity for considering the effect of force on motion.
• Suppose a light-weight vehicle (say a small car) and a heavy weight vehicle (say a loaded
truck) are parked on a horizontal road. We all know that a much greater force is needed to
push the truck than the car to bring them to the same speed in same time. Similarly, a
greater opposing force is needed to stop a heavy body than a light body in the same time,
if they are moving with the same speed. This will also help you on laws of motion.
• If two stones, one light and the other heavy, are dropped from the top of a building, a
person on the ground will find it easier to catch the light stone than the heavy stone. The
mass of a body is thus an important parameter that determines the effect of force
on its motion.
Force not only depends on the change in momentum but also on how fast the change is brought about. A seasoned cricketer draws in his hands during a catch, allowing greater time for the ball to stop and hence requires a smaller force.
I hope you got a hang up on newton's second law of motion. Probably on the next lesson let me help you understand on newton's third law of motion.
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