Momentum is a property of any moving object. It depends on both its mass and its velocity: momentum = mass × velocity, or p = m × v. Momentum is measured in kilogram metre per second, written kg m/s, with mass in kg and velocity in m/s.
Because velocity is a vector, momentum is a vector too. It has the same direction as the velocity. If you choose one direction as positive, an object moving the opposite way has negative momentum. A heavy lorry and a light car at the same velocity have different momentum, and doubling the velocity of an object doubles its momentum.
As a worked example, a car of mass 1200 kg moving at 15 m/s has momentum 1200 × 15 = 18000 kg m/s. The equation can be rearranged to find the velocity, v = p ÷ m, or the mass, m = p ÷ v. A trolley of mass 3 kg moving at 2 m/s has momentum 6 kg m/s. Momentum is important in collisions because the total momentum of the objects is conserved.