If a bowling ball and a ping-pong ball were traveling in straight lines at the same velocity, it would be harder to stop the bowling ball or change its path. This is because it has a larger mass, and, thus, a greater Inertia of Motion. If a bowling ball and a ping-pong ball were both standing still, it would be harder to make the bowling ball move. This is because it has a larger mass, and, thus, a greater Inertia of Rest. It is useful to remember that even very small objects may be very hard to stop if they are moving at a very high velocity, or have a very high Inertia of Motion .
Isotopes are atoms of the same element that vary in the number of neutrons in their nuclei.
They could be defined as atoms with the same atomic numbers but differing mass numbers , or atoms with the same numbers of protons but differing numbers of neutrons. See Nuclides . Isotopes will generally undergo the same chemical reactions, have the same chemical characteristics and physical characteristics. Isotopes will have differing nuclear qualities. Some isotopes of an element may be radioactive and others may be non-radioactive.