June Moment of Science:
Rolling Resistance

This summer Discovery Cube encourages you to roll outdoors with SPEED – Science in Motion at our LA campus and Hands-on Harley-Davidson in Orange County.  Both of these exhibits focus on vehicles: from race cars to motorcycles, all while teaching how these motorized, wheeled machines get us to our destination.  For this month’s Moment of Science, let’s experiment with different surface materials to understand why cars or street bikes run best on smooth asphalt.

First, gather your materials:

  • Piece of sturdy cardboard or wood (size really isn’t all that important, but you want it to be at least 4” wide and 12” long)
  • Stack of hard cover books that add up to about 4” high (again, total height isn’t all that important, but you don’t want it to be too much higher or shorter than 4”)
  • Roll of wax paper
  • Roll of foil
  • Small ball or balls, such as tennis ball, baseball, bouncing ball, etc.
  • Masking tape
  • Stop watch
  • Measuring tape or ruler
  • Scrap paper
  • Pencil

 

Then, follow these instructions:

  1. Start by creating your ground surface. Place the top of the wood or cardboard near the edge of the stack of books.  Your set-up should look something like this:
    books
  2. Place a piece of masking tape on the ground at the bottom of your ramp. This will tell you where to position the ramp if it moves during your experiment.
  3. Next practice holding and releasing (not pushing) your ball of choice from the top of the ramp. To do this, place the ball in the same spot near the top of the ramp, holding it in place (you can put a piece of masking tape just above where you want the ball to start each time so you are consistent).  Swiftly lift your hand, allowing the ball to roll down on its own.  Note: if the ball seems to roll down the ramp too quickly or slowly, adjust the height of the ramp by adding or removing a book or books.  Once you have the technique for releasing the balls down, you are ready to begin.
  4. Create a chart to record your data. It might look something like this, but you can set it up however you would like:
    chart
  1. While holding the stopwatch, time how long it takes for the ball to travel down the ramp and stop. Then measure the distance the ball traveled.  Record both in your chart.
  2. Repeat the previous step twice, so you have a total of three trials using the cardboard surface for your ramp.
  3. Cover your ramp with wax paper, folding the wax paper over the top and bottom of the ramp and taping it into place. Put the ramp in the same position you had it before (just behind the masking tape on the floor) and test how long it takes for your ball to travel down the ramp (and how far it travels before it stops) three times.  Record the data in your chart.
  4. Repeat this process a third time, but covering the ramp with foil.
  5. Average the amount of milliseconds required for the puck to slide down the ramp for each surface. Calculate the speed of the puck by dividing the distance traveled by the average number of milliseconds needed.
  6. Examine your results. Which surface did you initially think would be the fastest surface?  Which surface actually proved to be the fastest?
  7. Without changing the length or height of the ramp, how could you modify it further and continue your investigation? Can you find a way to put “pot holes” in your ramp?  Speed bumps?  Something that might represent gravel or sand?

For this experiment, we used the same ball (representing tires) on each surface.  While not all tires are the same, many people choose to put all-season tires on their cars year-round.  These tires are designed to work reasonably well in both summer conditions (hot, dry pavement) and winter conditions (wet, snowy, or icy roads).  By using the same ball on each surface, we were conducting a similar test to one where engineers test all-season tires on multiple surfaces.

Friction is a force that resists motion between two objects when they come in contact with one another.  But when tires on a car or motorcycle are moving on a road, the friction between the tire and the ground is only one of the forces at play.  Rolling resistance is the energy consumed by the tire per distance that the tire has covered.  Using physics, scientists know that replacing four tires in poor condition on a car with four new tires can increase the car’s fuel economy be as much as 7%.  But even easier (and cheaper) than replacing tires, make sure your tires are properly inflated.  Most of us drive cars everyday with underinflated tires.  Correct inflation can actually increase fuel efficiency by up to 8%.  Other fuel economy savings can come from smoother road surfaces (it takes more energy for your car to move over pot holes and rough surfaces).  Once you are done experimenting at home, check your tire’s inflation levels and come by the Cube to learn even more about motorized vehicles!