Sliding Science: How are Landslides Caused?
Have you ever seen a video of a landslide? Landslides are powerful geological events that happen suddenly, causing fear in people who live in areas with unstable hills, slopes and cliff sides. Each year in the United States, landslides can cause billions of dollars in damages, in addition to changing the environment and damaging the surrounding habitats. In this science activity, you will model landslides using a clipboard and pennies, and investigate how friction and the angle of a hill’s slope affects potential landslides.
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.
A landslide is any geological process resulting in a downward movement from a slope of rock, soil, artificial fill or a combination of the three, under the influence of gravity. Landslides can result from several causes, including mechanical weathering, rock disintegration due to physical or chemical activity, as well as erosion, earthquakes and volcanic activity.
One major force all landslides have in common that helps initiate them is gravity. We normally think of gravity pulling an object vertically down, but on a slope it gets more complicated. A force (like gravity) has a magnitude and a direction. On a slope, the effects of gravity can be separated into a component that’s parallel to the slope (pulling the object down the slope) and a component perpendicular to it (pulling the object against the slope’s surface). As the angle of the slope increases, gravity’s parallel component increases, and the perpendicular component decreases, causing less resistance for downward movement. This resistance is called friction and depends upon the perpendicular component of gravity and the slope’s and object’s surfaces. When the parallel component becomes greater than the perpendicular component, the object slides down the slope. This critical angle is called the angle of repose.
Extra: You could repeat this activity but use a protractor to quantify your results. What is the exact angle at which the different penny stacks start to slide down the slope? How do their angles of repose compare exactly?
Extra: Try this activity again but this time try making different sized penny stacks (with more or fewer pennies) and compare their angles of repose. How does the size of the penny stack affect how it slides down the slope?
Extra: Grab some other small objects and try repeating this activity. For example, you could make different objects from LEGOs. Do you get similar results?
Observations and Results
Did the tape-only penny stack usually start sliding down the clipboard first when you slowly raised the clipboard, increasing the angle of the slope?
The majority of the time the stack of pennies that were only coated in tape (and not a strip of paper towel) should have started sliding down the clipboard before the other stack of pennies did as the clipboard was raised up by its clip. For example, out of ten trials, the tape-only penny stack may have started sliding before the paper towel-wrapped stack in all ten trials. The resistance for downward movement on the slope is called friction, and it depends upon the component of gravity that is perpendicular to the slope, as well as the surfaces of the object and the slope itself. Because there is a greater amount of friction between two paper towel-coated surfaces rubbing against each other than there is between a paper towel-coated surface and a tape-coated surface, the penny stack with a paper towel strip on it had a greater amount of friction, or resistance to movement, when going down the slope. This greater amount of friction should have given the paper towel-coated stack a greater angle of repose compared to the tape-only stack.
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Teisha Rowland, PhD, Science Buddies
Science Buddies |
Slopes, angles, forces, gravity, friction
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