A laboratory experiment from the
Friction
Little Shop of Physics at
Colorado State University
CMMAP
Tug-of-War Reach for the sky.
Grade Level
• Activities designed to address
Kindergarten and 2nd Grade
Standards
Science Focus
• Pulling
• Pushing
• Friction
• Molecules
Time Required
• 15 - 20 minutes
In a game of tug-of-war, which side will win? It
depends on the force of friction.
Overview Necessary materials:
This exciting activity gives students a kinesthetic
experience playing tug-of-war with the usual force of • A long rope
friction, and then replaying the game with one team • An area long enough for a tug-of-war
wearing bags on their feet to reduce the force of game
friction. • Painter’s tape
• Plastic bags and pipe cleaners
Theory • A sense of fun!
If Galileo were still alive, we’d have to thank him for
pondering the force he named friction. It’s a very
important force to consider, and it’s everywhere. When two objects touch and slide against each other,
friction happens!
Let’s think about what is happening at a molecular level. When two objects touch, they both compress
and then push back on each other. For example, when you sit in your car, you are pushing down on the
car seat, and the car seat is pushing back on you. The tiny particles (molecules) that make up both you
and the car seat, compress and then push back, just like tiny springs. You explored this force, the normal
force, in the last activity, when your students worked with poppers and pullback cars.
The difference with friction, is that the two surfaces are sliding across each other as they touch. When this
happens, molecules tear apart! The molecules in each object don’t want to be pulled apart, so they pull
back, changing the motion of the two objects. The harder they push together, the more molecules get torn
apart. Let’s go back to the example of a person sitting in a car seat. When you get out of your car, you
slide across the car seat. You will feel more or less friction, depending on the fabric of the car seat, and
the fabric of your clothes. Some molecules are just “stickier” than others. I speak from experience! When
I get out of a car with leather seats and I’m wearing shorts, there is a strong friction force, and I feel like I
have to peel myself away from the seat. The molecules are really pulling back. When I slide out of a
vehicle with bench polyester seats, I tear so many molecules apart, that static charges build up in my
body, and I get quite a shock when I touch the gas pump (as do the gas station attendants)!
Doing the Experiment
Explain to your class that they are going to play a game of tug-of-war to learn more about forces. Choose
an even number of students to be on each side of the rope. Have students spread themselves out on either
side of the rope and pull lightly, so the rope is taut. Use painters’ tape to mark the floor at the center point
between team 1 and team 2. Explain that in the game of tug-of-war, one team must be able to pull the
other team across that center tape line, thus winning the game.
Have the students in your class predict whether team 1 or team 2 will win and have them share their
reasoning. They may say things like one team is stronger, or one team is bigger.
Now the exciting part! Begin the tug-of-war game and see who wins. There should be a lot of laughing
and fun. Discuss the results and their thoughts on why it worked that way.
Now have students think about the forces happening in the game. Have the same teams recreate the start
of the game, tugging gently on the rope, and then stop them in mid-action. Observe and think about what
their bodies are doing as they tug on the rope, and discuss with your class.
You may have to ask questions that help them focus on forces. What are you doing with your hands on
the rope? They are pulling on the rope. What pulls on you? Well, since we are pulling on the rope, the
rope pulls back on us. What are your feet doing? They are pushing and sliding on the floor. What does
the floor do? It pulls on you with friction!
Now have students on the winning team put plastic bags on both feet, and use pipe cleaners to tighten the
bags around their ankles. Have the students play another game of tug-of-war, and see who wins this time.
If you are wearing plastic bags on your feet, the rope wins and you lose. Discuss this new development
with your students and find out why they think this happened. They may say that the bags are slippery
which is true. Some molecules (your shoes) are stickier than other molecules (plastic bags). The stickier
molecules don’t want to tear apart, so they pull back with a bigger friction force. The plastic bags are less
sticky, so the friction force is less.
Summing Up:
This is a great activity to help students experience and really focus on forces, especially the force of
friction. We believe that thinking about friction at a molecular level helps everyone understand this
important force.
For More Information
CMMAP, the Center for Multi-Scale Modeling of Atmospheric Processes: http://cmmap.colostate.edu
Little Shop of Physics: http://littleshop.physics.colostate.edu
