Key concepts
Physics
Friction
Motion
Materials

Introduction
Have you ever tried to get a running start and slide across a smooth, wooden floor while wearing socks? What happens if you try the same thing on a carpeted floor or while wearing shoes? The amount of friction between your feet and the floor surface determines how well you can slide. Some combinations of surfaces, such as socks on a wooden floor, produce very little friction. Other combinations, such as rubber soles on a wooden floor, produce much more friction. In this project you'll do a much smaller-scale friction experiment by launching tiny objects along a surface with a rubber band. How do you think friction will affect how far the objects slide?

Background
Friction is the force that opposes motion between two surfaces that are touching one another. Friction is a very important part of everyday life—without it, you wouldn't be able to walk because your feet would just slip on the floor! Sometimes it is very important for surfaces to have high friction, such as the rubber tires of a car on the road. Other times we want surfaces to have low friction, such as when you go down a slide at the playground or sled on a snowy hill. The amount of friction changes depending on which materials come into contact with one another.

Another thing that can affect friction on an object is the object's weight. Imagine pushing two boxes—one heavier than the other—made out of the same material. You will have to push harder to move the heavier box. This is because gravity pulls down harder on the heavier one, which increases its friction with the floor. In this experiment you will keep the weight of the objects you use constant so that you can test different materials without worrying about gravity’s effects.

Materials

• Rubber band
• Stack of five or six quarters
• Scotch tape
• Smooth wooden surface, such as a table or floor (If you do not have a wooden surface, you can use any other type of smooth countertop or table.)
• Rough carpeted surface (If you do not have access to carpet, you can tape down several paper towels on top of your smooth surface.)
• Ruler (optional)

Preparation
• Stack your quarters on top of one another. Wrap them in tape so they are secured together. Make sure the bottom of the stack is smooth, with no sharp corners of tape sticking out. (These could get caught on the carpet.)
• Make sure the wooden and carpeted surfaces you will work with are free from any other objects or obstructions.

Procedure
• Form a "C" shape with your index finger and your thumb.
• Carefully stretch the rubber band between your index finger and thumb to form a slingshot.
• Still carefully holding the rubber band, turn your hand upside-down and touch the tips of your finger and thumb to the wooden surface so the rubber band rests just above the surface.
• Load the stack of quarters into your slingshot and pull back on the rubber band. Pay attention to how far you pull back the rubber band; it is important to pull back the same amount each time. What do you think will happen when you release the quarters?
• Let go of the quarters to launch them. Watch closely to make sure they slide across the wood and do not get launched into the air or tumble. If the quarters don't slide smoothly, adjust your rubber band and try again.
• Repeat the launch several times and watch how far the quarters go. Remember to make sure you pull the rubber band back the same distance each time. Do the quarters slide very far or do they come to a stop quickly? What do you think this tells you about friction between your stack of quarters and the wooden surface?
• Now, repeat the same process on the carpet. Be careful not to let any corners of the tape get stuck on the carpet. How far do the quarters go on carpet? Do they go farther or less far than they did on wood? What does this tell you about friction between the stack of quarters and the carpet? Is it higher or lower than on wood?
• Extra: Use a ruler to record how far the quarters go each time. Record all your results in a table and then calculate an average distance for each surface. How far do the quarters go on average for the wood surface? How far do they go on average for the carpet?
• Extra: Try out more test surfaces in addition to wood and carpet. What if you go outside and try the experiment on a hard, rough surface, such as the sidewalk or driveway? What about other surfaces you can find in your home? How far do the quarters slide on different surfaces? Can you guess whether a surface will have high or low friction just by looking at it?
• Extra: Repeat the activity using different objects instead of your stack of quarters. For example, how do your results change if you try the experiment with a large rubber pencil eraser? Remember that an object's weight determines how much friction it will encounter, so try to use objects that are about the same weight. (You can use a kitchen scale to weigh different objects.) Can you find an object that always slides farther than the quarters? What about one that always doesn’t slide as far?

Observations and results
Did the quarters go farther on the smooth wooden surface or on the carpet?

You should have found that the quarters went much farther on the smooth wooden surface than they did on the carpet. Depending on the strength of your rubber band and how far back you pulled it, you might even have launched them all the way off the table or countertop! This occurs because there is much more friction between the stack of quarters and the carpet’s rough surface than there is between them and the smooth surface of the wood. Because there is less friction slowing them down, the quarters can slide farther on the wood before they eventually come to a stop.

More to explore
Effect of Friction on Objects in Motion, from Science Buddies
Friction Basics, by Andrew Rader at Physics4Kids
Fight Slippage with Friction, from Scientific American
Science Activities for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies