It is conserved in both elastic and inelastic collisions,
 and its change in a given time interval is the impulse.
In quantum mechanics, the Fourier transform of this quantity's operator is represented by the position
 operator, and this quantity's operator is equal to negative i times h bar times the gradient. Translation
 invariance of the Hamiltonian or Lagrangian implies conservation of this quantity, which is equal to the
 product of h bar and the wave number for a photon. It is conserved in both elastic and inelastic collisions,
 and its change in a given time interval is the impulse. Its time derivative is equal to the product of mass and
 acceleration by Newton's Second Law. For 10 points, identify this product of mass and velocity.
In quantum mechanics, the Fourier transform of this quantity's operator is represented by the position
 operator, and this quantity's operator is equal to negative i times h bar times the gradient.
linear {momentum} [{do} not accept "angular momentum"]