GCSEC: Non Round Rollers Label

NON ROUND ROLLERS

Place any two of the roller assemblies on the track so they are about six inches apart. Place the plastic board on top of them. Space your hands about twelve inches apart on the board. Now try rolling the board from side to side.

Try several different combinations of the roller assemblies.

Make sure the rubber bands are on the wheels, and then try rolling the "car" on the table.

Each of the shapes is what might be called a constant width figure. Drawings showing how the shapes were created are included later in the label.

If you measured across the non circular rollers at different places you would get differing readings. However, if you measure from contact point to contact point when the roller is between the plastic and the table top, you will always get 3.5".

These rollers demonstrate that measuring across a "circle" and different points and getting the same diameter guarantees that the circle really is round.

Submarines travel deep into the ocean where the water pressure is intense. If the submarine hull cross section is not perfectly circular it will be crushed. As this exhibit demonstrates, circularity cannot be guaranteed merely by measuring the diameter at several different places. When manufacturing submarines, full sized half circle patterns are held up to the outside of the frame and hull to guarantee that the submarine will be perfectly circular.

Drawings showing how these shapes were created are on the next page. You can easily draw similar shapes at home and cut them out of cardboard or Styrofoam to make your own non round rollers.

Why does the same shape allow the plastic to roll perfectly flat but make the back of the car go up and down?

The plastic is on top of the roller and the roller has a constant width. The car is riding on the axle of the roller assembly. The distance from the axle to the edge of the shape changes a lot as you move around the edge of the roller.

Why do you think the rubber bands on the car's rear wheels are necessary?

When the plastic board is on top of the wheel, you are pushing down, or vertically, causing a lot of friction between the plastic and the roller and between the roller and the table. When you push the car most of your force is forwards and backwards, or horizontal. There is less friction, so the rear wheels tend to slide. The rubber bands greatly increase the friction of the wheels with the table, and they roll rather than slide.