• In the preceding observations, the vector astronaut, once he is out of the spaceship, is zero. By the first law of motion the acceleration of the astronaut is zero. ⊳ . NEWTON’S SECOND LAW OF MOTION 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 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.