Inertia

Inertia is a concept used to explain the tendecy of an object to remain at rest or in motion (constant speed) unless a force is applied. The mass of an object is a measurement of inertia. Inertia can also be described as the resistance to the change of momentum of a body. Therefore, an object with greater inertia (mass) is more difficult to change its momentum than an object with lesser inertia (mass).