Galileo theorized that in the absence of any other force, a moving object will continue to move. He called this resistance to changes in motion, inertia. Sir Isaac Newton later polished this concept – turning it into his first law, the Law of Inertia. From his Principia: Every object continues in a state of rest or of uniform speed in a straight line unless acted on by a nonzero net force. In other words, if an object is moving in a direction, it will continue to move in that direction unless another force is exerted upon it. Likewise, if an object is in a state of rest, the law of inertia requires the object to stay in that state unless another force is exerted upon it.
In the following dramatization, your students will see an extreme example of Newton's First Law in action! We aren't sure if this REALLY occurred without some help from Hollywood, but it certainly gets the point across.
NOTE: This movie was proven "fake" on Mythbusters, but it's still a fun way to demonstrate Newton's law of inertia.
Check out the products from Arbor Scientific below for the Inertia Apparatus – a great way to demonstrate Newton's First Law – and other items that illustrate other Newtonian Laws!