Effect of Friction on Objects in Motion

Objective

The goal of this project is to investigate how far equally-weighted objects with different surface textures will slide when propelled across surfaces with different textures.

Introduction

Friction is a force between objects that opposes the relative motion of the objects. In this project, you will be studying kinetic friction (also called sliding friction). When two objects are moving relative to one another, kinetic friction converts some of the kinetic energy of that motion into heat. You can feel the heat of kinetic friction if you rub your hands together.

The same thing happens when two objects are sliding past one another—for example, when you push a box across the floor. Part of the energy of your pushing moves the box, and part of the energy is lost to kinetic friction. How much energy is lost? What factors do you think will act to increase or decrease kinetic friction?

Think about what happens if you rub your hands together. If you press your hands together, you have to push harder to slide your hands past each other, and your hands heat up more quickly. Pressing your hands together is like adding more weight to the box before trying to slide it across the floor. The added weight makes the box push down harder on the floor, and you will have to push harder on the box to make it slide.

Think about what happens if you rub your hands against a smooth, polished surface, like wood furniture, compared to a surface with a rougher texture, like denim cloth. Which surface produces more kinetic friction?

The goal of this project is to investigate how the texture of surfaces affects the amount of kinetic friction produced when objects move across different test surfaces.

Terms, Concepts and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

kinetic (sliding) friction,
forces.

More advanced students should also study:

Netwon's laws of motion,
normal force,
coefficient of friction.

Questions

How is friction produced?
What effect does friction have on the speed of a rolling object?
What types of surfaces will produce the most friction when they rub against one another? What types of surfaces will produce the least amount of friction?
When you want to go down a slide at the playground, you first have to climb up a ladder, working against gravity to get to the top. When you slide down, only part of the energy of your climb goes into the speed of your slide. What happens to the rest of the energy of your climb?

Bibliography

This webpage has a good introduction to friction:
Darvill, A., date unknown. "GCSE Physics: Energy, Forces & Motion: Friction," Broadoak Community School, Weston-super-Mare, England [accessed November 8, 2006] http://home.clara.net/darvill/enforcemot/friction.htm.
Wikipedia also has an article on friction:
Wikipedia contributors, 2006. "Friction," Wikipedia, The Free Encyclopedia [accessed November 8, 2006] http://en.wikipedia.org/w/index.php?title=Friction&oldid=87445377.
More advanced students should also study Newton's three laws of motion, which are introduced in these four lessons from The Physics Classroom:
Henderson, T., 2004. "Newton's Laws," The Physics Classroom and Mathsoft Engineering & Education, Inc. [accessed November 8, 2006] http://www.physicsclassroom.com/Class/newtlaws/newtltoc.html

Materials and Equipment

To do this experiment you will need the following materials and equipment:

various surfaces (with different textures) to test, e.g.:
- wood,
- felt,
- aluminum foil,
- sandpaper,
- or some other surface—use your imagination!
- Note: you'll need enough material to cover your test area. Objects (see below) will be propelled over the test surface with rubber-band power.
objects (with different textures) to test, e.g.:
- wood block,
- plastic block,
- sponge,
- rubber eraser,
- or some other object—use your imagination!
weights (e.g., coins, washers, etc.) to equalize mass of objects,
gram scale or homemade balance (to make sure objects have equal mass),
rubber band,
rubber band launcher:
- For small objects, this can be your hand, inverted, with the rubber band stretched between your thumb and forefinger. Use a ruler to make sure that the distance between your thumb and forefinger is the same each time.
- For larger objects, you'll need to stretch the rubber band between two rigid supports at the level of your test surface.
- In both cases, use a ruler to measure how far back you stretch the rubber band when launching the objects so that you are consistent for each test.
ruler,
tape measure.

Experimental Procedure

Cover the test area with the surface material to be tested.
Set up your rubber-band launch station at one end of the test area.
With the object in contact with the test surface, pull back on the rubber band to some measured distance. Use the same amount of stretch for each object. This insures that the launching force will be the same for each test object.
Launch the object horizontally, so that it slides over the test surface. If the object does not stay in contact with the test surface, try again. You may need to use less force, or adjust the height of the rubber band above the surface.
Measure and record the distance the object travels.
Test each object at least 10 times (more is better).
Calculate the average distance each object travels. More advanced students should also calculate the standard deviation.
Make a bar graph showing the average distance traveled (y-axis) vs. surface combination (test surface and object). Arrange the bars in order of increasing average distance traveled.
Which surface combinations produced the most kinetic friction?
Which surface combinations produced the least kinetic friction?
Can you explain your results in terms of the physical properties of the materials you tested?

Variations

What happens if you hold the test surface and the object constant, but change the weight of the object (by attaching progressively more weight on top of the object)? Make a graph of distance traveled vs. object weight under these conditions.
Use a spring scale to measure the force needed to drag various objects across different surfaces. Record both the transient force needed to overcome static friction (Figure 1, left), and the maintained force necessary to counteract sliding friction (Figure 1, right). How do these two forces vary with different surfaces? How do these forces vary with the weight (normal force) of the test object? How do these forces vary with the surface area of the test object?
Figure 1. Using a spring scale to measure static friction (top) and sliding friction (bottom) of an object.

Credits

Andrew Olson, Ph.D., Science Buddies

Sources

This project is based on:

Okita, N.A., 2003. "Effect of Friction on Objects in Motion," California State Science Fair Abstract [accessed November 8, 2006] http://www.usc.edu/CSSF/History/2004/Projects/J0226.pdf.