The mass of an object can be loosely described as the quantity of matter in it. The greater the mass of an object, the greater will be the force required to start it moving from rest or to change its speed if it is already moving.
Mass is measured in units of kilograms (kg) in the SI metric system or pounds (lb) in the Imperial and Federal systems. Unfortunately, the same names are commonly used for the units of weight (which is a force), and this causes a great deal of confusion, as will be explained a little later under the heading Units. In this book, we will always use kilograms for mass, and newtons for weight.
The quantity that decides the difficulty in stopping a body is its momentum, which is the product of its mass and the velocity of movement.
A body having a 20 kg mass moving at 2 m/s has a momentum of 40 kg m/s, and so does a body having a 10 kg mass moving at 4 m/s. The first has the greater mass, the second the greater velocity, but both are equally difficult to stop. A car has a larger mass than a bullet, but a relatively low velocity. A bullet has a much lower mass, but a relatively high velocity. Both are difficult to stop, and both can do considerable damage to anything that tries to stop them quickly.
To change the momentum of a body or even a mass of air, it is necessary to apply a force. Force = Rate of change of momentum.