Slippery Solids

1. Rub your thumb and index finger together. How does it feel to rub your skin together? How easily do the surfaces of your fingers slide against each other?


2. Now try it again to test a few different materials. Notice how the materials feel between your fingers as they slide.
   • Hold some paper bits between your fingers.
   • Hold some pieces of chunky glitter between your fingers.
   • Color a piece of paper with a pencil, then press both fingers in the colored area to cover your fingers with graphite.


3. Each test uses materials that are made up of flat, small particles--even the graphite from the pencil, where the particles are too small to see. You’ll measure the slipperiness of these materials by testing how well they help a gummy bear slide down a ramp.


4. Get your ramp ready, using a binder or other materials. Tape a piece of uncut paper to the top of the ramp. Place your ramp on the baking tray.


5. Put one gummy bear at the top of the ramp. Does it slide down? Try it with two other gummy bears and record your results on the data sheet.


6. Draw two lines from the top to the bottom of the ramp to divide your surface into three equal lanes. Sprinkle paper bits all over the first lane, chunky glitter (about 2 teaspoons) all over the second lane, and color the third lane with pencil (the darker the better).


7. Based on your observations in step 2, record your predictions on the data sheet. Do you think each material will help the gummy bear slide?


8. “Coat” the back of each gummy bear by pressing or rubbing it into the material covering each lane, then place the bear at the top of the ramp. Does it slide? Test each lane 3 times and record your results on the data sheet. Did your results match your predictions? Which slid fastest?


9. Explore further! Look around for other materials that might help the bear slide. Test and record your results on the additional rows of the data sheet.



Clean-up:

Wash your hands with soap to remove graphite from your fingers. Take apart your ramp and throw away used paper, glitter, and gummy bears.

Materials that help two surfaces slide past each other are called “lubricants.” Lubricants are placed in between two surfaces to reduce friction and heat. You might be familiar with liquid or semi-solid lubricants, like oil or grease, that are often used to make cars and other machinery run smoothly. In this experiment, you observed the performance of a different type of lubricant--layered materials. Without any lubricants, there is too much friction between the gummy bear and the ramp for the bear to slide down. By adding paper bits or chunky glitter in between, you created layers of these particles that could easily slip past each other. Graphite is a common industrial lubricant that works in the same way, but its flat, layered particles--each just one atom thick--are too small for our eyes to see. Did you try any other layered materials that worked?

When you pull layered materials apart into individual layers, each sheet is so thin that you see unexpected behaviors. Think about your observations with the gummy bears. Using thin layers of paper, glitter, or graphite as a solid lubricant helped your gummy bear slide. In the real world, too, solid lubricants are used as coatings to help moving parts slide against each other. You can find them in trains, rockets, and other places where moving parts are exposed to extreme temperatures, dust, or moisture--conditions where liquids just don’t work as well. But aside from being slippery, layered materials have other cool properties too. Thin layers can lead to faster chemical reactions and better flow of electricity. Researchers are inventing new ways of making layered materials to help create batteries and fuel cells for longer-lasting, non-polluting energy.

Sometimes science is so amazing, it feels like magic! Linxi Xu, Pratibha Mahale, and Jeff McNeill get excited by their innovative experiments with layered materials. They’re even more excited that their work could lead to a healthier planet and a better future for all of us.

Below are suggested alignment between this activity and concepts in the Next Generation Science Standards.

Performance Expectations

• 2-PS1-2: Analyze data obtained from testing different materials to determine which materials have the properties that are best suited for an intended purpose.


• 3-PS2-1: Plan and conduct an investigation to provide evidence of the effects of balanced and unbalanced forces on the motion of an object.


• MS-PS2-2: Plan an investigation to provide evidence that the change in an object’s motion depends on the sum of the forces on the object and the mass of the object.

Disciplinary Core Ideas:

PS1.A: Structure and Properties of Matter

2nd Grade

• Different properties are suited to different purposes.

PS2.A: Forces and Motion

3rd Grade

• Each force acts on one particular object and has both strength and a direction. An object at rest typically has multiple forces acting on it, but they add to give zero net force on the object. Forces that do not sum to zero can cause changes in the object’s speed or direction of motion.

Middle School

• The motion of an object is determined by the sum of the forces acting on it; if the total force on the object is not zero, its motion will change. The greater the mass of the object, the greater the force needed to achieve the same change in motion. For any given object, a larger force causes a larger change in motion.


• All positions of objects and the directions of forces and motions must be described in an arbitrarily chosen reference frame and arbitrarily chosen units of size. In order to share information with other people, these choices must also be shared.

Please click on the PDF below for a more detailed description of how this activity ties to NGSS