Slippery Slopes and Sticking Surfaces: Explore the Forces of Friction
AbstractWhoopee! No matter what age, who doesn't like flying down the slides at parks and pools? In this experiment, you might be surprised what you can learn about the fascinating forces of friction while sliding down (or sticking to) those fun, slippery slopes. Caution: only speed demons need apply for this activity . . .
SourcesThe idea for this project came from this DragonflyTV podcast:
- TPT, 2006. Luge by Jenn and Emily, DragonflyTV, Twin Cities Public Television. Retrieved on January 15, 2018.
ObjectiveThe goal of this project is to use a slide to learn about the forces of friction.
Frictional forces surround us. We rub our hands together to generate heat when we're shivering. Screeching tires burn rubber on the road when cars start too quickly or turn too sharply. Meanwhile, special treads on those same tires cling to the road to keep us safe when we travel icy highways. And who hasn't been grateful at least a few times for the traction exerted from rubber-soled running shoes or water sandals in slippery situations?
We all recognize what friction is, but do you really understand what causes it? This project focuses on friction, its causes and forces and specifically how it affects how fast you can slip down slides. To get you started, check out the project video. It shows how two talented athletes, Emily and Jenn, explored ways to increase speed down a slide. They applied their results to similar tests in their favorite winter sport, the luge. Few of us have Olympic-quality luge runs readily available in our neighborhoods, or can handle the 65 mph speed down a narrow icy chute, so we provide directions on how you can use a standard neighborhood slide to run your experiments on friction.
In the video, Emily and Jenn focused on different techniques to get a quicker push from the top of their luge runs. Ultimately, it's the frictional interactions all along the run that really influence travel speed down to the bottom. Understanding the physics and molecular forces that determine why things slip or grip as they move across surfaces is the goal of this science project.
We most commonly think of friction in terms of surface roughness, like the resistance of two pieces of sand paper catching between our fingers. But there are also small electromagnetic interactions of atoms and molecules sitting on the surface of even smooth objects that are important in generating friction. These tiny but strong molecular snags are especially apparent between very smooth surfaces like two pieces of plate glass that stubbornly stick to each other if not separated by paper or sheets of plastic. In general, when two objects interact they produce some level of frictional force upon each other determined by their weights and the combination of surface roughness and intermolecular sticking. How much friction the two surfaces produce determines whether the objects move or simply remain in place.
A couple of basic laws of physics describe the forces of friction. You'll learn more about these principles as you complete your background research and run your experiments. But in simple terms, when an object is stuck on a surface, like a heavy box you are trying to push along the floor, scientists describe this as a "static" friction interaction because there is no movement. The weight of the box essentially holds it against the floor and the friction generated between the box and floor's surface is greater than the pushing force trying to move it (you). When your buddy comes along to help you out, the pushing force now becomes greater than the static frictional force and the box slides easily along. However, there still is some resistance from the box because it remains in contact with the floor surface; this moving type of friction is called "kinetic" friction. In general, static friction is greater than kinetic friction and that explains why it sometimes takes a shove to get a heavy box moving, but once it's unstuck, it's easier to keep it sliding along the floor.
In this project, a local playground slide serves as your test site for experimenting with static and kinetic friction. You and a friend will explore how to change the frictional forces that can slow you down or speed you up when you push off and slide down. You'll time your runs and experiment with different materials (e.g., pillowcase, towel, cardboard, carpet, plastic bag) to find out how each "speed mat" interacts with the slide to affect your starts and total times. While we've offered suggestions for the types of materials to try in your experiments, no doubt you'll be able to come up with a few ideas of your own. You'll have to take careful measurements with a stopwatch in these experiments because there might only be a fraction of a second difference between some runs and others. But as Jenn said in the video, "that can make all the difference between winning and losing a race." So small differences add up to big results when you are talking about friction.
Good luck, have fun, and watch out below!
Terms and ConceptsTo do this project, you should do research that enables you to understand the following terms and concepts:
- Static friction
- Kinetic friction
- Newton's third law of motion
- Rolling and sliding friction
- Coefficient of friction
- Normal force (N)
- Vector diagrams of friction force and net force
- How is friction defined scientifically?
- What are the differences between static and kinetic friction?
- What are the causes of friction?
- What is Newton's Third Law of Motion and how does it relate to friction?
- What factors related to friction come into play when you sit on top of and move down a slide?
- Introductory site with simple definitions of terms related to friction:
Kurtus, R., (2006). Resistive Force of Friction. School for Champions. Retrieved April 28, 2007.
- Basic introduction to a few concepts and mathematical relationships between friction, coefficients of friction, and normal force:
Nave, C.R., (2005). Friction. HyperPhysics, Department of Physics and Astronomy, Georgia State University. Retrieved April 28, 2007.
- Good website for high school level lessons on physics and how to use vector diagrams to represent the forces acting on an object:
Henderson, T., (2004), Force and Its Representation. The Physics Classroom. Retrieved April 28, 2007.
- This project is based on:
TPT, (2014, March 5). Luge by Jenn and Emily. DragonflyTV, Twin Cities Public Television. Retrieved Janujary 15, 2018.
Materials and EquipmentTo do this experiment you will need the following materials and equipment:
- Neighborhood slide, preferably straight or not too curved (longer is better),
- Stop watch that shows time to 1/10 of a second,
- A helper to time your runs,
- At least three of the following "speed mats" or test materials to slide on (slides can be made of many different materials—plastic, metal, or concrete, for example—choose speed mats appropriate to your particular slide):
- Pillowcase (cotton, silk, or flannel)
- Bath towel (maybe one washed in fabric softener and one not)
- Small carpet or scatter rug
- Large, plastic garbage bag
- Flattened piece of cardboard or heavy butcher paper
- Other material you'd like to test
- Find a slide to use for your study—the longer the better. Decide which materials you want to use for speed mats in your experiment. Select at least three you think will give you different results.
- You should make two different observations for each mat you use in your tests:
- The Push Off (Static Friction): Note how much effort it takes to get going down the slide. Try to keep your push offs as consistent as possible (where you place your hands, how many times you push, the position of your legs, etc). This will be a "qualitative" measurement. That means you won't assign a number to it, but instead give a description about each push like an "easy" push was all that was needed to get you moving, or "medium", "hard", or "extra effort" was required.
- The Speed (Kinetic Friction): Note the time it takes to get from the top to the bottom of the slide. It's likely that the differences between your tests will be in fractions of a second, so your friend will have to be very good about making careful measurements with the stopwatch. They should start the clock right when you push off, and they should stop the watch the moment they see and hear your feet hit the ground.
- Tips: You might try a few trial runs with no mat to practice taking times before you get into your real tests. Also, be sure to not rush too much as you go down the slide so you don't risk falling off or hurting yourself.
- Do at least ten runs for each type of mat (more trials would be even better). Record a description of the start and the total time for each run in your notebook.
- Calculate the average time for each mat material you used in your tests.
- Make a table that shows the average time for each material as well as your description of each start.
- Do you see any differences in the times or ease of start for your materials? Were some materials faster or slower by a small or large margin?
- Were your results what you expected or were you surprised? If so, how?
- For more advanced analysis, make vector diagrams showing the direction and relative magnitude of the friction forces that illustrate the results for each of your test materials.
- For help with data analysis and setting up tables, see Data Analysis & Graphs.
- For a guide on how to summarize your results and write conclusions based on your data, see Conclusions.
Ask an Expert
- Pick one of the fastest materials from your initial experiment to try these variations.
- Size of contact surface area: sitting vs. lying down; legs straight ahead or tucked onto the mats; taller person vs. shorter person of similar weight.
- Weight of slider: compare two people of very different weights.
- Temperature: try your experiment on a hot day and then repeat it on a cold day.
- Moisture: dry, sunny day versus a foggy day or just after a rain.
- If you use a concrete slide, try it with and without loose sand on the surface of the slide.
- If you have multiple slides, made from different materials (e.g., plastic, metal, concrete), you can extend your experiment to include the different slides.
- For a more quantitative experiment on friction, see the Science Buddies project Effect of Friction on Objects in Motion.
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