I went to a science teacher boot camp looking for magic tricks. This is not as far fetched as it may seem. Many physical phenomena look like magic. Leaves change color. Clouds move across the sky. Magnets stick to refrigerators.
Much of the early part of the program was gosh-wow that called for special chemicals. Instant Snow, Gel beads, dye tablets. I like to do magic with common items. I was having fun, but not seeing anything I wanted to show my grandchildren.
Then we got to Inertia. We started simply enough. Set an empty plastic soda bottle on the table, mouth side up. (be sure the cap is not on the bottle.) Place a business card on top of the mouth. Place a small hex nut on top of the business card. Be sure the hex nut is half the size of the mouth, or smaller. Now, curl your index finger against your thumb, so the finger nail of the index finger is touching the top joint of your thumb. Now flick the index finger straight out. The thumb will slow you down and when the index finger finally does release, it does so with force. Practice this a few times.
Then flick your index finger against that business card.
The 1st law of physics states that an object at rest will stay at rest unless it is acted upon by outside forces. That hex nut is at rest. That hex nut has mass. For all practical purposes, mass and inertia are the same thing. Bottom line, in our environment, gravity (which depends on the attraction of masses)pulls things towards the center of the Earth. When you flick that business card out from under the hex nut, there's a competition. Will the hex nut fly away on the business card, like a wizard on a flying carpet, or will it fall to the bottom of the empty bottle when its supporting business card is no longer holding it up? We picked the hex nut because it has a lot of mass for its volume. Yes, that's what is known as density. And yes, this is way easier to see than to read.
Do it. I'm not going to tell you what happens.
It's a good demonstration but it lacks the gosh-wow factor.
Then our instructor, put a table cloth on the table. He placed china dishes and glasses on top of the table cloth. He filled the glasses with liquid. And he yanked the table cloth out from underneath. I'd seen this done before, and not thought that I was watching a demonstration of inertia. There are two tricks. 1) the table cloth must not have a hem. 2) You have to pull the table cloth edge straight down towards your feet -- not back towards your belly. This is going to be a great addition to my magic show. It's powerful magic with the potential for making a mess. What kid can resist? I hope my grandchildren take this to school with them.
There were other variations, stacking an egg on a toilet paper roll, on top of a pie tin on top of a glass of water. Putting the hex nut inside a balloon. Putting a penny inside a balloon. But for sheer magic -- I'll take that table demo.
Here's a variation, in case my words weren't sufficient: Steve Spangler did this on the Ellen show.