Sometimes I wonder if we can send our students to the International Space Station (ISS) for a homestay. With gravity and friction out of the way, it would be obvious that all a mass wants to do, is to be stick to its current motion. No need for any teaching, every student understands immediately that maintaining motion requires no force.
Better still, throw the student out of the ISS. With nothing to push/pull against, the student immediately grasps that his current motion cannot be changed without an unbalanced force.
This reminds me of the scene in the movie “Mission to Mars”, where the crew were forced to abandon ship and take refuge in the REMO (“Resupply Module”) orbiting Mars. The plan was for Woody to launch himself at the module and attach a tether to it.
It was Woody’s inertia that killed him. Unable to conjure any force to act on himself, he was doomed to carry on moving at his current velocity. (In the movie it was suggested that eventually Woody was accelerated by Mars’ gravity and burnt up as he sped through the Martian atmosphere.)
Back on Earth, with gravity and friction clouding our mind, we form the misconception that forces are required to maintain motion. Worse still, when we confuse relative motion with motion, we make up phantom forces to fit our misconception. The “jerking force” that throws you backward when your taxi jolts off when the light turns green? The “braking force” that throws you forward when taxi brakes suddenly? And the “centrifugal force” that crushes you against the car door when the taxi swerves suddenly? These are all fictitious forces. Actually, it was just your inertia. Your body was only trying to keep to its current state of motion.
Watch the video below. See whether you can explain the motion of the tissue box on the dashboard.