Bearings

Stand on a cookie sheet or cutting board which is placed on the floor (find a smooth floor with no carpet). Ask someone to gently push you across the floor. Notice how much friction they feel as they try to push you.

Want to make this job a bit easier?

Here’s what you need:

[am4show have=’p8;p9;p11;p38;p92;’ guest_error=’Guest error message’ user_error=’User error message’ ]

• two boards (about 12″ x 12″, or whatever you have handy)
• 4-10 dowels (or round, not hexagonal, pencils)
• handful of marbles

Now place three or four dowels parallel about six inches apart between the board and the floor. Smooth wooden pencils can work in a pinch, as can the hard cardboard tubes from coat-hangers. Ask someone to push you. Is there a direction you still can’t travel easily? Now let’s add another direction to your motion…

Replace the dowels with marbles. What happens? Why do the marbles make you do in all directions? What direction(s) did the dowels roll you in?

 
Download Student Worksheet & Exercises

BONUS EXPERIMENT IDEA! To really drive this point home, you can make your own low-friction ball bearings: Get two cans (with a deep groove in the rim, such as paint cans) and stack them. Turn one (still on top of the other) and notice the resistance (friction) you feel. Now sandwich marbles all along the rim between the cans. Place a heavy book on top and note how easily it turns around. Oil the marbles (you can spray with cooking spray, but it is a bit messy) and it turns more easily yet.

Exercises 

1. Why do the marbles make you go in all directions?
2. What direction(s) did the dowels roll you in?

[/am4show]