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Develop and expand STEM explanations through friction fun

During this experiment, you will investigate friction by rubbing your hands together in various ways. Before starting the experiment, write a prediction statement about what you think will happen or what you think you will notice.

I predict that ______________________________________________________________________________________

Now begin the experiment. Rub your hands together, palm to palm, back and forth at least 20 times. Write down what you notice. What do you hear? What do you feel? Can you quantify any of this experience?

________________________________________________________________________________________________

________________________________________________________________________________________________

Now try several variations of this experiment and note the results. You may want to increase the speed or pressure or rub the palm to the back of the hand or add lotion or soap. Document any similarities and differences in the chart below. You may transfer the notes above into the first row of the chart as your starting point.

Describe rubbing action Observed results

Rub hands together 20 times at 1 time per second, palm to palm. Moderate pressure.

Summarize what you discovered during this brief experiment:

Did your findings match your prediction made before you started?

HO_C2S_FrictionFun–Apr 03, 2014

extension.psu.edu/youth/betterkidcare

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Copyright ©2014 The Pennsylvania State University

This publication is available in alternative media on request.

Claudia C. Mincemoyer, Ph.D., Better Kid Care Program Director 341 North Science Park Road – Suite 208, State College, PA 16803 Web site: extension.psu.edu/youth/betterkidcare

In words that a younger child would understand, explain the “why” behind what was happening as you rubbed your hands together.

If you had more time, what other variations would you like to try or how might you expand this experiment with friction to other areas?

Friction factsFrictional forces are everywhere. People rub hands together when they’re shivering. Screeching tires burn rubber when cars start too quickly or turn too sharply. Special treads on those same tires cling to the road to keep passengers safe on icy highways. Rubber-soled running shoes or water sandals prevent slipping. Often people recognize friction but may not understand or be able to explain what causes friction.

When two objects (like two hands) interact, they produce a force called friction. The objects can be a solid (like skis on snow), a gas (like air slowing a car) or a liquid (like a river resisting a boat). The amount of force created is based on the items’ weights and a combination of the surface roughness and the “intermolecular” sticking between the two. Picture the resistance of two pieces of sandpaper snagging on each other’s surfaces. That kind of catching happens both at the visible level and at the microscopic level. The rougher the surface, the more friction is produced. Typically, two solid items create more friction than two liquid things. Depending on the friction produced, objects may move easily or may not budge. More friction is produced when objects are pushed together harder like when a person pulls extra hard on a brake to stop a bike. Heavier objects also produce more friction than lighter objects because the gravitational pull on the heavier object is stronger, holding it more tightly to the surface.

When an object is stuck on a surface, like a heavy box that a person is trying to push along the floor, scientists call this a “static” friction because there is no movement. The weight of the box essentially holds it against the floor and the friction generated between the box and floor’s surface is greater than the pushing force trying to move it. When more pushing force is added, it overpowers the static frictional force and the box slides. However, there still is some resistance from the box because it remains in contact with the floor; this moving type of friction is called “kinetic” friction. In general, static friction is greater than kinetic friction and that explains why it sometimes takes a shove to get a heavy box moving, but once it’s unstuck, it’s easier to keep it sliding along the floor.

When a person or an object works to overcome static friction, some of the energy put into the process is converted to heat. Picture the reaction when a person rubs two stones together until a spark is produced. On the microscopic level, the heat is actually the movement of the atoms in the objects moving or vibrating. The more friction overcome, the more heat produced. The faster the atoms move in the object, the more heat produced. Lubricants (like oils or soaps or waxes) can be used to reduce surface roughness and friction and consequently reduce heat buildup.

References: Science Buddies. 2013. “Slippery Slopes and Sticking Surfaces: Explore the Forces of Friction.” Accessed March 19, 2014. http://www.sciencebuddies.org

eHow. “Why Does Rubbing Your Hands Together Make Them Warmer?” Accessed March 19, 2014. http://www.ehow.com/how-does_5410709_rubbing-hands-together-make-warmer.html#ixzz2t28JZcLT

Kidepede – History and Science for Kids. 2014. “Friction.” Accessed March 19, 2014.http://scienceforkids.kidipede.com/physics/machines/friction.htm#sthash.TDRoAF2H.dpuf