Inertia

'Inertia is the resistance of the object to any change in its motion, including a change in direction. An object will stay still or keep moving at the same speed and in a straight line, unless it is acted upon by an external unbalanced force.

For example, a ball lying on grass will not start moving unless a force, such as a kick, acts on it. If the ball is already rolling, it will tend to keep moving in the same direction. On Earth, however, it usually slows down because of forces such as friction and air resistance. Other examples of inertia include a person moving forward when a car stops suddenly, or an object sliding farther on ice because there is less friction.^[1] The same idea can be applied to motion: an object in motion will stay in motion unless some outside, opposing force acts upon it. Inertia is also called Sir Isaac Newton's First Law of Motion. The First Law of Motion says that: "Every body perseveres in its state of being at rest or of moving uniformly straight ahead, except insofar as it is compelled to change its state by forces impressed. [Cohen & Whitman 1999 translation] This basically means: Every object stays at rest or stays moving at the same speed unless something makes it change.Johannes Kepler gave inertia its name in Epitome Astronomiae Copernicanae (published in three parts from 1617 to 1621)

    Albert Einstein developed the idea of Special Relativity based on Galileo's statement that it is impossible to tell the difference between a moving object and a stationary one without some outside reference to compare it against.

• Moment of inertia
• Newton's laws of motion