SLIDING ON FRICTION

Friction is important to help us move.  When we walk, our feet push on the ground, without friction, they would just slide on the ground, and we'd probably fall down.  The same thing applies to cars, without friction, the tires would just spin round and round, instead of moving the car.

On the other hand, there are lots of times when friction gets in the way.  Too much friction makes equipment wear out too quick.  Another thing too much friction does is slow things down from moving well.  

Have you ever been on a slide that didn't go fast?  That's because there's too much friction.  Although your weight is pulling you down the slide, the friction of your body or clothes against the surface of the slide is slowing you down.  Let's see if we can find out how to overcome this friction.

You Will Need:

• Two 12" wood rulers, or two wood strips about the same size

• A stack of books

• Some tape (masking tape is best)

• Two quarters

• A piece of aluminum foil (slightly larger than the ruler)

• A piece of plastic grocery bag (slightly larger than the ruler)

• A piece of waxed paper (slightly larger than the ruler)

• A piece of sandpaper (slightly larger than the ruler)

• A piece of fabric (slightly larger than the ruler)

• A piece of notebook paper (slightly larger than the ruler)

• Chart and a pencil to record your test results.  Click here for the printable chart

ANGLE AND SPEED

Let's see if the angle of the slide will effect how fast the slides are.

1. Attach one end of both rulers to the table with tape.
   

2. Place a five inch high stack of books under the other end of one ruler.  This one will be our "control" ruler, we will leave it alone, and compare everything else to it.  
   

3. Place a three inch stack of books under the free end of the other (test) ruler.
   

4. Place one quarter at the top of each slide, holding them with your hands.  Release both quarters at the same time.  Watch to see which one reaches the bottom first.  Record your results.
   

5. Add books to the three inch stack under the test ruler until it is eight inches tall.
   

6. Repeat the race with the two quarters.  Watch to see which one reaches the bottom first this time.  Record your results.
   

MATERIAL AND SPEED

Now let's see if the material we use will effect how fast our slides work.

1. Cover the "test" ruler with one of the six test materials (aluminum foil, plastic grocery bag, waxed paper, sandpaper, fabric and notebook paper).  Make sure the top surface of the ruler (the slide side) is very smooth.  Tape the test material to the bottom side of the test ruler so that it cannot slip off.  Be sure that you don't change the "control" ruler in any way.
   

2. Tape the "test" ruler, covered with the test material, to the table, next to the "control" ruler.  Make sure that the bottoms of the rulers are even, and that they are both on the same stack of books, so that they are at the same angle.
   

3. Place one quarter at the top of each slide, holding them with your hands.  Release both quarters at the same time.  Watch to see which one reaches the bottom first.  Record your results.
   

4. Repeat steps 1 through 3 for each of the test materials.  Record your results each time.
   

LOOKING AT THE RESULTS

The angle that the rulers were placed at should have made a difference in how fast the quarters went down the "slides."  That's because at a steeper angle the weight of the quarter is pushing it down the slide, while at a shallow angle, the weight of the quarter is pushing it into the slide.  When the ruler is flat, all the weight of the quarter is pushing into the ruler, adding friction, which slows down movement.  As the angle gets steeper, less and less of the weight of the quarter is pushing into the ruler.  This reduces friction, and allows the quarter to move faster.

As you raise the angle of the slide, the quarter should move faster.  This works the same with slides in the playground, or water park.  Steeper slides should be faster than slides that aren't as steep.  Of course, that doesn't take into account the material that the slides are made of.

In the second set of tests, the angle was the same, but the material of the slide changed.  You probably found that the smoother materials, aluminum foil, paper, and especially waxed paper were faster than the rough materials, cloth and sandpaper.  

Smooth, hard materials almost always have less friction than rough, or soft materials.  That allows the slide to work faster.  To get the fastest possible slide, you'd want to use the smoothest material, and put the slide at the steepest angle.

EXPLAINING HOW IT WORKS

Friction comes about as a result of the contact of any two surfaces.  The smoother the surfaces are, the less friction there is.  However, even surfaces that seem smooth aren't as smooth when viewed through a microscope.  If we look at aluminum foil through a very powerful microscope for example, we would find that the surface of the foil looks like a landscape full of flat rocks!  Paper is even worse, it would look as rough as a sea of logs laid side by side.  

Of all the materials we tested, the wax paper would probably appear the smoothest in our microscope.  But, even that, wouldn't look totally flat and smooth.  It would look like a series of milky white sand dunes.

When the surfaces slide against each other, these microscopic ridges I'm talking about bump into each other.  Even though we can't see it, and can't feel each individual bump, we can feel their effect.  The more of these bumps there are, and the bigger they are, the more "drag" we feel on the surface.  That makes the slide go slower.

When engineers design equipment, they take this drag into account.  Although rubbing two pieces of metal against each other a couple of times doesn't seem to damage them, rubbing them against each other thousands, or even millions of times will.  That's what happens to most of the things we use every day.  Over the years, they rub against each other missions of times.

To prevent things from wearing out, engineers try to use materials that have very little friction, especially in places where they will rub against other parts.  Plastics are very good for this, especially a type of plastic called Teflon®.  Another way they prevent friction is by using lubricants such as oil in a car engine.  Lubricants fill in the microscopic bumps and pits in the materials, making the surface smoother than it would normally be.

So, now that you've tried different materials for a slide, what do you think the best material, for the least friction, would be?  Maybe you should try using these materials under you the next time you go to a playground, just to see if they really work.