Momentum-Definition

Linear momentum is a vector, the product of velocity and mass:

= m · 

Therefore the unit of momentum is the product of the units of mass and velocity, kg$\cdot$m/s.

Because the velocity is a vector, so is the linear momentum. They always point in the same direction. In case that you have forgotten, let us remind you that the mass is just a constant that the velocity vector gets multiplied by, and the rule for this is then that each component of the velocity vector gets multiplied with the mass to obtain each component of the momentum vector:

If:     = (v_x, v_y),     then:     = (p_x, p_y) = (m·v_x, m·v_y)

From the rules for calculating with vectors, then we also have the equation for the relation between the absolute magnitudes of the momentum and velocity:

Examples:

1. If a car and a truck both travel down the freeway at 65 mi/hr, and the truck has 8 times the mass of the car, how does the momentum of the car compare with the momentum of the truck?
   • The truck has 8 times the momentum of the car.
2. If a wide receiver with mass 75 kg runs with a speed of 10 m/s, and a 110 kg line-backer runs with a speed of 6.5 m/s, who has the larger momentum?
   • p_wide-rec. = (75 kg) (10 m/s) = 750 kg m/s
   • p_lineback. = (110 kg) (6.5 m/s) = 715 kg m/s
   • So the wide receiver has the higher momentum.

© MultiMedia Physics, 1999