Slip Sliding Away: Experimenting with Friction
AbstractAs you headed up the mountain to enjoy your last ski trip, you may have noticed a sign reading: Hazard! Icy Roads Ahead—Put On Your Chains. Putting chains on car tires increases the resistance between the tires and the road allowing the car to "grip" the road. This resistance to sliding is called friction. In this experiment, you will be investigating how to increase and decrease the friction between two surfaces.
Compare friction between dry and icy surfaces by measuring slip angle and slip height.
When driving a car down a road, the friction that allows the car to move is between the car's tires and the road. Friction is the resistance to motion when two objects rub together. Friction normally allows a car to "grip" the road. However, when the road is icy, the friction between the two surfaces decreases and this may allow the car to slip.
To understand the difference between situations where there is high friction or low friction, think about whether it would be easier to ride your sled down an icy snow-covered hillside or down a rough gravel driveway. It is easier to ride your sled down the icy hillside because both the runners on your sled and the icy hillside are smooth and slide past one another with little resistance. This is an example of low friction. On the other hand, riding your sled down the gravel driveway is difficult due to the rough surface of the driveway resisting the motion of the sled. This is an example of high friction.
As you can imagine, friction partly happens when the rough parts of an object catch the rough parts on another object as they rub together. The friction that occurs depends on a value called the coefficient of friction. This is a measure of how much two objects interact with each other when rubbing together, and this depends on what the material the two objects are made of. The friction that takes place also depends on the mass of the moving object (and gravity). In other words, the heavier the object, the greater the friction. All of these factors play a role in determining the friction that takes place between two rubbing objects.
In this science project, you will investigate how friction affects how well an object moves down an icy surface compared to a dry surface.
Terms and Concepts
- Coefficient of friction
- What is friction?
- How can friction be overcome?
- When is high friction beneficial? When is it not beneficial?
- When is low friction beneficial? When is it not beneficial?
- Carr, Karen, Portland State University. (2011, October 24). Friction. Retrieved February 10, 2012.
- Rader's Physics4kids.com. (n.d.). Motion Basics: Friction. Retrieved February 10, 2012.
Introduction to General Physics Concepts:
- Hewitt, Paul G, 2002. "Conceptual Physics," Prentice Hall, IL.
Simple Physics Concepts for Kids:
- Keller, R.W., 2005. "Real Science for Kids: Physics, Level 1," Albuquerque, NM: Gravitas Publications, Inc.
Materials and Equipment
- 2×4 wooden boards cut to 0.5 m length, or about 20 inches (2 boards are needed). Note that 2×4 wooden boards are 2 inches by 4 inches.
- Small plastic tub or jar with lid
- Metric ruler
- Lab notebook
- Soak one piece of wood in water and freeze overnight.
- Fill the plastic tub with sand and close the lid.
- Build a ramp using several books and the piece of wood.
- Determine the ramp height and ramp angle at which the tub of sand first slips down the ramp.
- Place the tub of sand onto the ramp.
- If it slips down the ramp, remove the tub, lower the ramp height and try again.
- If it does not slip, remove the tub, raise the ramp height and try again.
- Make your changes in height small enough so that you can accurately determine the ramp height and ramp angle just high enough to allow the tub to slip down the ramp.
- At the angle that the tub first moves, measure the height of the book pile with the meter stick (in centimeters [cm]) and the angle of the ramp with the protractor (in degrees [°]). Record your results.
- Conduct the same experiment (steps 2–5) with the icy wood.
- For any experiment, it is important to do multiple trials to assure that your results are consistent. Repeat steps 2–5 for at least three separate trials for each surface, and record your results. It will be easier to keep track of your results if you write them down in your lab notebook in a data table like Table 1 below.
- Calculate the average slip angle and the average slip height for each condition (the dry wood and the icy wood). Record your results in your data table.
- When you are done testing, look at your results and try to draw some conclusions. How does the friction compare between the dry and icy surfaces based on your average slip angles and average slip heights?
|Sample Data Table|
|Average Slip Angle
|Average Slip Height
Ask an Expert
- Conduct the same experiment with two dry wooden boards and increase the friction of one of the boards by covering it with sandpaper. How does the increased friction on the board affect how well the tub slides down it?
- Cover the surface of one piece of wood with oil or foil to decrease the friction. How does this change how the tub slides down the wood compared to sliding down wood that is not covered with oil or foil?
- Instead of changing the surface of the wood, you can try changing the bottom of the tub by rubbing it with oil or covering it with sandpaper. How does this change how well the tub slides down the ramp compared to a tub that has not been changed?
- If you want an advanced challenge, you can try to calculate the angle of the ramp. If you know the height of the ramp and the length of the ramp, you do not actually need a protractor to figure out the angle of the ramp. All you need is to do some math. The equation used to solve the angle of the ramp is sin(theta)=opposite/hypotenuse. "Theta" is the angle, "opposite" is the ramp height, and "hypotenuse" is the ramp length. Find out more about how to do this calculation at Finding an Angle in a Right Angled Triangle from Math is Fun Advanced. Does the calculated angle match the one you measured with your protractor?
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