Force can be a push or pull that either changes or does not change an object's motion depending on whether the net force is balanced or unbalanced. Unbalanced forces will cause changes to an object's velocity due to inertia, the tendency of objects to resist changes in motion, with more inertia for objects with greater mass. Isaac Newton's first law of motion states that objects at rest will stay at rest and moving objects will stay in motion with constant velocity unless acted upon by an unbalanced force, such as friction between surfaces.