Key Concepts
Friction, motion
A coin being pulled back on a rubber band on carpet.

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, like socks on a wooden floor, produce very little friction. Other combinations, like 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 with a rubber band. How do you think friction will affect how far the objects slide?

This activity is not appropriate for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.

Background

Friction is the force that opposes motion between two surfaces that are touching each other. 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, like the rubber tires of a car on the road. Other times, we want surfaces to have low friction, like when you go down a slide at the playground or go sledding on a snowy hill. Different materials can have very different friction when they come into contact with each other.

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

Materials

  • Rubber band
  • Stack of 5 or 6 quarters
  • Scotch tape
  • Smooth wooden surface like 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.
  • Optional: ruler

Preparation

  1. Stack your quarters on top of each other. Wrap them in tape so they are all stuck 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).
  2. Make sure the wooden and carpeted surfaces you will work with are free from any other objects or obstructions.

Procedure

  1. Form a "C" shape with your index finger and your thumb. 
  2. Stretch the rubber band between your index finger and thumb to form a slingshot.
  3. 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. 
  4. 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. 
  5. 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.
  6. 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?
  7. 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. Write all of your results down 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 rough surface like 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 experience, so try to use objects that are about the same weight. You can use a kitchen scale to check how much different objects weigh. Can you find an object that always slides farther than the quarters? What about one that always slides less 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 rough surface of the carpet than there is with the smooth surface of the wood. Since there is less friction slowing them down, this allows the quarters to slide farther on the wood before they eventually come to a stop.

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Credits

Ben Finio, PhD, Science Buddies

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Key Concepts
Friction, motion
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