On a smooth table, place a non-breakable plate and several non-breakable glasses filled with water on a smooth tablecloth with no hem. Grab one end of the tablecloth, and ask your students what will happen if you pull the tablecloth away. Pull the tablecloth away -- this should leave the plate and glasses in approximately the same position. Ask your students why the plate and glasses stayed in place. Introduce the concept of inertia, that objects at rest stay at rest unless a force acts on them, and objects in motion remain in motion unless a force acts on them. Discuss how inertia explains how the items stayed in place.
Repeat the tablecloth demonstration, this time with a rough fabric in place of the smooth tablecloth. The plates and cups should be pulled off the table -- this is why it’s particularly important that they be non-breakable. Review with students the definition of inertia, and ask why they think the cups and plate moved this time. According to the definition of inertia, it must be because some force acted on them. Introduce the concept of friction, the force of resistance that occurs when two bodies rub against each other. Now pass around the two cloths, so that the students can feel that one is rougher than the other, and produces more friction when pulled.
Ask your students what happens when you roll a ball; rather than going on forever, it eventually stops. Ask students to consider why it stops when it does. Have students brainstorm ways that the ball could keep rolling further. For instance, if you rolled it harder, or if the surface you rolled it across were smoother, it might move further. Your hand setting the ball in motion is an example of a force. The ball’s contact with the surface it’s rolling across is another force -- friction. Ask students to imagine what would happen if you rolled the ball across a perfectly smooth surface.
Cut equal-length strips of sandpaper, wax paper and construction paper and glue them side by side on a piece of corrugated cardboard to make three different “tracks.” Prop one end of the cardboard on a stack of books, forming a ramp, and tape the cardboard in place. Review with your students the concepts of friction and inertia. Show students a toy car, and ask them to predict which track the toy car will roll down the fastest, and why. Place the toy car at the edge of one of the tracks, and time it as it rolls down using the stopwatch. Repeat for each track. Compare your results to the class’s predictions.