Newton's First Law and the linear air track

 

I wanted to tell my youngest class that forces do 3 things: they change shapes of objects and change direction and speed of movement. I decided to use the linear air track for the latter two. The air track removes the friction so the glider travels at constant speed between the ends. The action of forces on the elastic bands changes the direction. I could have added that that the shape of the elastic changes. They don't know yet about hysteresis loops in the elastic but this dissipative process means that after each impact the speed visibly decreases. In between impacts, Newton's First Law applies.