An \(80-\mathrm{kg}\) physicist and a friend are ice-skating. The physicist, distracted, collides from behind at \(7.0 \mathrm{~m} / \mathrm{s}\) with his friend, who is skating at \(5.0 \mathrm{~m} / \mathrm{s}\) in the same direction. After the collision, the physicist continues in the same direction at \(4.0 \mathrm{~m} / \mathrm{s}\), but his friend is now moving (still in the same direction) at \(8.0 \mathrm{~m} / \mathrm{s}\). What is her inertia?