Every moving object has momentum. It tells you how hard it is to stop the object. A slow lorry and a fast cyclist can both be difficult to stop, but for different reasons: the lorry has a large mass and the cyclist has a large velocity. Momentum takes both into account.
Momentum is calculated with the equation momentum = mass × velocity, or p = m v. Momentum, p, is measured in kilograms metre per second (kg m/s). Mass, m, is in kilograms (kg) and velocity, v, is in metres per second (m/s).
For example, a 2 kg trolley moving at 3 m/s has a momentum of 2 × 3 = 6 kg m/s. A car of mass 1200 kg moving at 15 m/s has a momentum of 1200 × 15 = 18000 kg m/s.
Because momentum is mass multiplied by velocity, doubling the velocity of an object doubles its momentum if the mass stays the same. Velocity has a direction, so momentum has a direction too, the same as the direction of the velocity. An object that is not moving has no momentum.