Newton’s Law of Inertia states that
“An object at rest will remain at rest until it is acted on and an object that is moving will remain moving in the same direction and at the same speed unless another force acts on it.”
To demonstrate this, we messed around with some tealight candles and talked about what force was making the candles move, and what force made them stop. You don’t need candles though – you could use anything that has a bit of weight to it; like a stone, or a pencil sharpener, or whatever you can find lying around the house. We happened to have a big bag of tealights and they seemed a good shape and size, so we used those.
First we put the tealights on the table and gave them a shove. They traveled across the table, and then they stopped. We talked about why the candles moved in the first place, and why they stopped instead of just keeping on going forever.
It was the force of us pushing that made them move, and it was the force of the friction between the table and the candles that made them stop.
The next thing we tried was to balance a piece of card on a cup, and put a candle on top.
When we pushed the card slowly off the cup, we saw that the candle went with it; but when we pushed the card very quickly, the card moved, but the candle didn’t go with the card, it fell into the cup instead.
This happened because there were different forces acting.
When we pushed the card slowly, the friction between the candle and the card was stronger than the force of the pushing, so the friction caused the candle to move with the card. When we pushed the card quickly, however, the force of the push was stronger than the friction between the candle and the card, meaning that the friction was overpowered, so the card moved, but the candle stayed where it was.
According to the law of inertia, the candle would have stayed there hovering in the air above the cup, unless another force was acting on it. We realized that it was the force of gravity that made the candle then fall down into the cup.