ABSTRACT
At the end of this chapter you should be able to: • define momentum and state its unit • state Newton’s first law of motion • calculate momentum given mass and
velocity • state Newton’s second law of motion • define impulse and appreciate when impul-
sive forces occur • state Newton’s third law of motion • calculate impulse and impulsive force • use the equation of motion v2 = u2 + 2as
in calculations
The momentum of a body is defined as the product of its mass and its velocity, i.e. momentum = mu, where m = mass (in kg) and u = velocity (in m/s). The unit of momentum is kg m/s
Since velocity is a vector quantity, momentum is a vector quantity, i.e. it has both magnitude and direction. Newton’s first law of motion states:
Hence the momentum of a body remains the same provided no external forces act on it. The principle of conservation of momentum for a closed system (i.e. one on which no external forces act) may be stated as:
The total momentum of a system before collision
of the system after collision in the same direction. In Figure 12.1, masses m1 and m2 are travelling in the same direction with velocity u1 > u2. A collision will occur, and applying the principle of conservation of momentum:
total momentum before impact = total momentum after impact
i.e. m1u1 + m2u2 = m1v1 + m2v2 where v1 and v2 are the velocities of m1 and m2 after impact.
