Changes

==Explanation==

==Explanation==

The first law defines an [[inertial]] [[frame of reference]] as one which is acted upon by no outside forces.  In general, inertial frames are far easier to understand conceptually and deal with mathematically than accelerated frames.

The first law defines an [[inertia|inertial]] [[frame of reference]] as one which is acted upon by no outside forces.  In general, inertial frames are far easier to understand conceptually and deal with mathematically than accelerated frames.

The second law relates force and [[momentum]].  Mathematically, <math>F = dp/dt = d(m*v)/dt = m*dv/dt + v*dm/dt</math>.  Usually <math>dm/dt=0</math>, so the law is simplified to <math>F = m*dv/dt = m*a</math>, or mass times acceleration.  A notable exception is [[rocket]] motion, where <math>dm/dt</math> is not 0, and so <math>F = m*a</math> does not apply.  Note that the quantities '''F''', '''p''', '''v''', and '''a''' are all [[vector]] quantities--that is, they have an associated direction as well as a magnitude.  In general, the second law gives a way to predict the motion of an object by summing all the forces acting on that object.

The second law relates force and [[momentum]].  Mathematically, <math>F = dp/dt = d(m*v)/dt = m*dv/dt + v*dm/dt</math>.  Usually <math>dm/dt=0</math>, so the law is simplified to <math>F = m*dv/dt = m*a</math>, or mass times acceleration.  A notable exception is [[rocket]] motion, where <math>dm/dt</math> is not 0, and so <math>F = m*a</math> does not apply.  Note that the quantities '''F''', '''p''', '''v''', and '''a''' are all [[vector]] quantities--that is, they have an associated direction as well as a magnitude.  In general, the second law gives a way to predict the motion of an object by summing all the forces acting on that object.