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.
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.
- Clear tape
- Eight pennies
- Hard clipboard
- Paper towel
- Ruler (optional)
- Take a piece of tape a little longer than the length of four pennies lined up next to each other (about 3.5 inches long) and set two pennies on the tape so the pennies are touching, side by side.
- Set one penny on each of the pennies on the tape so that you have two stacks of pennies, with two pennies in each stack. Then wrap the tape long-ways completely around the pennies so that they are held in place, still stacked and side by side. The tape should slightly overlap on the top side.
- Repeat this with four other pennies so that you have made two taped stacks of pennies like this.
- Cut out a strip of paper towel that is slightly longer than the length of one of the stacks of pennies, and the same width as the pennies (in other words, the paper towel strip should be about two to 2.5 inches long and almost one inch wide).
- Take one of the taped penny stacks and make sure the rough, exposed tape edges are on the top (and the smooth side is on the bottom). Then, using two small pieces of tape, tape the paper towel strip long-ways on to the stack of pennies so that both edges of the strip curve around to the top side and are taped there. Do not put any tape on the bottom side, which should be completely covered by the paper towel strip.
- Set a clipboard on a flat surface. Clip a paper towel sheet on to the clipboard. (If you cut a rectangle out of the sheet, put the rectangle at the bottom.)
- Place both penny stacks you made on the clipboard so that they’re both touching the clip at the top. They should be touching the clip long-ways and not be touching each other.
- Make sure both stacks are placed so that their rough tape edges are facing up, and the paper towel strip or smooth taped side of the stacks is down, touching the clipboard. How does the bottom of each stack feel compared to each other? Is one much smoother than the other?
- Holding on to the clipboard’s clip, slowly and steadily lift that end of the clipboard (make sure the opposite side stays down, touching the flat surface). Which stack of pennies slide down the clipboard first as you increase its angle? Stop tilting the clipboard as soon as one of the stacks of pennies starts to slide down it.
- Repeat this process at least nine more times, for a total of ten trials. Each time be sure to start with the clipboard laying flat on a flat surface and with both stacks of pennies sitting next to each other by the clip. Also make sure to slowly lift the clipboard each time. For each trial, which stack of pennies slide down the clipboard first? Are your results fairly consistent?
- If one stack of pennies usually slid down the clipboard first, why do you think this happened? Why do you think the angle of repose (the angle at which an object slides down a slope) may have been different for the two different stacks of pennies? What do you think your results might have to do with friction?
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.
Ask an Expert
- Landslide Types and Processes, from the U.S. Geological Survey
- Mine Landslide Triggered Earthquakes: Record-Breaking Slide Would Bury Central Park 66 Feet Deep, from ScienceDaily
- Friction Basics, from Rader’s Physics4Kids.com
- Fun, Science Activities for You and Your Family, from Science Buddies
- Landslides: What Causes a Hill to Become Creep-y?, from Science Buddies