Inertia is a property of matter that causes it to resist changes in velocity (speed and/or direction). According to Newton's first law of motion, an object with a given velocity maintains that velocity unless acted on by an external force. Inertia is the property of matter that makes this law hold true.
The amount of inertia that an object possesses is proportional to its mass . However, inertia is not the same thing as mass or momentum (the product of velocity and mass). The mass of an object can be measured by observing the extent of its inertia. This is done by measuring the amount of force required to produce a certain acceleration .
The property of inertia is useful in navigation systems. A massive object can provide a constant reference for acceleration, and in particular, changes in direction, because it tends to maintain a constant orientation in space. Inertial guidance systems are used in aircraft, spacecraft, oceangoing vessels, and missiles.