Your friend says that inertia is a force that keeps things in their places, either at rest or motion.Do you agree? Why or why not?

Inertia is a property of matter, as described in Newton's first law of motion, that basically says if not external force is applied to an object, the object will continue with the motion it already has. That means that objects at rest stay at rest, and objects in motion stay in motion. The greater the mass, the greater the inertia. That is why it takes so long, for example, for a large ship or train to stop or change direction versus stopping or changing the direction of a toy car.  You also experience inertia when you are driving in a car and change speeds suddenly.  Your body wants to keep moving at the same rate as before and so your seat belt may suddenly engage as your body tries to keep moving forward.  Newton talked about inertia more than mass when describing the relationship between the force applied to objects and the resultant acceleration the object would undergo.