According to second law the rate of change of linear momentum of a body is directly proportinoal to the net external applied force. The change in momentum takes place in the direction of the force. The law gives a relation between force and momentum. It also gives a quantitative definition or measure of the force.
Mathematically,
F = dp/dt = d/dt (mv) = m\(\frac{dv}{dt}\) (for a system with constant mass)
= ma
\(a=\frac{dv}{dt}\) = Instantaneous acceleration of body.
Remember
1. For a constant mass and changing velocity
\(F=m\frac{dv}{dt}\)
2. For constant velocity and changing mass.
\(F=v\frac{dm}{dt}\)
3. In terms of rectangular coponents of the force and momentum
\(F_x=\frac{dp_x}{dt};F_y =\frac{dp_y}{dt}\) and Fx \(=\frac{dp_x}{dt}\)
4. A force applied along X–direction does not affect the momenta of the body along Y and Z directions and Fy does not affect px and pz . Similarly Fz does not produce any change in px and py .
