We’ve all seen this classic stage magic trick: You arrange a nice table setting, with plate and cup and silverware and maybe a nice vase of flowers, on a pretty silk tablecloth. Then you yank the tablecloth out from underneath, but the dishes all stay on the table! We even have a video clip of it here.

 Newton’s First Law of Motion states that an object’s velocity is constant unless there is a net force acting on it. What this means is that if an object is not moving (at rest), it will not start moving until there is a force pushing or pulling on it. If an object is moving at a constant speed and direction, it will keep going with that same speed and direction unless a force pushes or pulls on it to change that. An object’s inertia is its resistance to changing its velocity, e.g. how difficult it is to start it moving from rest.

So that’s what’s going on here. We are applying a force to the tablecloth, pulling it away. But so long as we have a smooth, unwrinkled tablecloth, we’re not applying any force to the dishes, just the cloth. So the cloth moves away quickly, but the dishes stay where they were. This is a very popular way of demonstrating inertia, and you can find it on our website.

The dishes do have a force pulling straight down on them, of course – the force of gravity. They aren’t actually moving downwards because they can’t go through the table. Technically, the table and tablecloth exert an upward force on the dishes, which is formally called the normal force (“Normal” here is using an archaic definition that means “perpendicular to the plane,” not what we usually mean by normal). The normal force here is equal and opposite to the force of gravity, counterbalancing it and canceling it out, so the dishes don’t move. When the tablecloth goes away, the dishes are for a moment not touching anything, so gravity does pull them down a very short distance, the thickness of the tablecloth, and they hit the table with a clatter. But the point is that they don’t follow the tablecloth off the table.

But there can be some more complex issues at work here. Why does it matter that the tablecloth is smooth and unwrinkled? Why do you have to pull quickly on the tablecloth, and not slowly? Why do we sometimes see this trick fail, and end up with the dishes all breaking on the floor? (Please don’t do that.) Perhaps the physics is more subtle than it first appears. Click here to read more about the secrets of this magic trick!