Facilitator/Educator Guide: Landslides: What Causes Rocks to Slip and Slide?
Landslides are powerful geological events that happen suddenly, striking fear in people who live near unstable hills, slopes, and cliffs. In this classroom activity using tape, pennies, a paper towel, and a clipboard, you will help students model landslides and investigate how friction and the angle of a hill's slope affects them.
Landslides cause an average of 25 to 50 deaths annually in the United States and billions of dollars in economic losses, in addition to changing the environment and damaging surrounding habitats. 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 force of gravity. Landslides can result from several causes, including mechanical weathering (the gradual disintegration of rock from physical activity), chemical weathering (the gradual disintegration of rock from chemical activity), erosion (the removal of rock or soil by wind, water, or other natural processes), earthquakes, and volcanic activity.
Landslides occur in five main types (slides, flows, falls, topples, and lateral spreads) and while these can behave differently from each other, they all have one major force in common that helps initiate them: gravity. We normally think of gravity as pulling an object vertically down, but when that object is on a slope, it gets a bit more complicated. A force (like gravity) has both a magnitude and a direction. On a slope, the effect of gravity can be separated into a component that is parallel to the slope (pulling the object down the slope) and a component that is perpendicular to it (pulling the object against the slope's surface).
|Figure 1. For an object on a slope, the force of gravity can be separated into two components: one parallel to the slope and one perpendicular to it.|
As the angle of the slope increases, the parallel component of gravity increases, and the perpendicular component decreases, leading to a smaller resistance for downward movement. This resistance is called friction and depends upon the component of gravity that is perpendicular to the slope, as well as the surface materials of the slope and the object (or mass) being pulled. When the parallel component becomes greater than the friction holding it in place, the object slides down the slope. This critical angle is called the angle of repose.
In this classroom science activity, students will model a landslide with a clipboard and pennies by making two objects with different surfaces to investigate how the angle of repose changes when friction changes. How will the friction between an object and a slope affect the object's angle of repose?
This science activity can serve as a starting point for a variety of science and geology discussions. Here are a few examples of questions that can be used to start a discussion:
- How does gravity cause an object to move downhill?
- What is an angle of repose? What factors might affect it?
- How does friction between two objects affect how the objects move?
- How do you think landslides made of solid objects move compared to landslides made of fluid-like mud?
Needed for preparing ahead:
- Transparent tape (1 roll)
- Optional: Metric ruler or tape measure (1)
- Pennies (8 for each demo or small group)
- Paper towel (1 sheet for each demo or small group)
- Scissors (1 pair)
Needed for each demo or small group at the time of the science activity:
- Stacks of four pennies taped together (2). One stack should have a paper towel strip on it.
- Clipboard, hard (1)
- Paper towel (1 sheet)
|Figure 2. You need only a few simple household materials to do this fun science activity.|
What to Do
Prepare Ahead (10-20 minutes)
- Take a piece of tape a little longer than the length of four pennies lined up next to each other (about 9 centimeters [cm] long) and set two pennies on the tape so the pennies are touching, side by side.
|Figure 3. Set two pennies side by side on a piece of tape (about 9 cm long).|
- Put one penny on each of the pennies on the tape so that you have two stacks of two pennies each.
|Figure 4. Stack one penny on each of the pennies on the tape.|
- Then wrap the tape longways 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.
|Figure 5. Fold the tape over the two stacks of pennies.|
- Repeat this with four other pennies so that you have made two taped stacks of pennies like this.
- Cut a strip of paper towel slightly longer than the length of one of the stacks of pennies, and the same width as the pennies (the strip should be about 5 to 6 cm long and 2 cm wide).
|Figure 6. Cut a strip of paper towel slightly longer than the stack of pennies and just as wide.|
- Take one of the taped stacks of pennies and make sure the rough, exposed tape edges are on the top, and the smooth side is on the bottom. Then, fold the paper towel strip longways on the stacks of pennies so that both short edges of the strip curve around the top side, where you will tape them together using two small pieces of tape. Do not put any tape on the bottom side, which should be completely covered by the paper towel.
- You should end up with one stack of pennies surrounded by tape, and one stack with a strip of paper towel on it.
|Figure 7. For each demo or small group, you should have two stacks of pennies, one with just tape and another with a paper towel strip on it.|
Science Activity (< 10 minutes)
- Each classroom demo or small group should have two stacks of 4 pennies taped together (one with a paper towel strip), a hard clipboard, and a paper towel sheet (this can be the same one from which you cut a strip to cover the pennies).
- Explain to students that the two stacks of pennies represent different types of rocks (or other particles) in a landslide, where some rocks have more friction when rubbing against the hill than other rocks do. The "hill" the pennies will slide down will be represented by a hard clipboard with a paper towel on it. Students will investigate which stack of pennies has the lowest angle of repose, or the lowest angle the stack needs to start sliding down the clipboard.
- Have students set the clipboard on a flat surface. Ask them to clip the paper towel on the clipboard. (If there is a strip cut from the sheet, have them position it at the bottom.)
- Next ask students to place one of the penny stacks on the paper towel-covered clipboard, touching the clip at the top. Now have students place the other penny stack next to the first stack, also touching the clip. Both stacks should be placed so that the rough tape edges are facing up, with both the paper towel on one stack and the smooth taped side of the other stack touching the paper towel on the clipboard.
|Figure 8. Have students place the two stacks of pennies on the paper towel-covered clipboard, touching the clip.|
- Holding on to the clip, have students slowly and steadily lift the clipboard, making sure to lift only the side where the clip is. Ask them to stop tilting the clipboard as soon as one of the stacks of pennies starts to slide down. Have students note which stack of pennies slid down first. If you want, ask students to write down their results.
|Figure 9. Have students slowly lift the clipboard, tilting it up from the clip, and watch to see which stack of pennies slides down first.|
- Have students repeat this process at least nine more times, for a total of ten trials. Each time they should start with the clipboard lying flat on a flat surface, and with both stacks of pennies sitting near each other by the clip. Make sure students slowly lift the clipboard each time, and note which stack of pennies slid down the clipboard first. If you want, ask students to write down their results.
- You can ask students to discuss their results, and why the angle of repose may have been different for the two different stacks of pennies, and how this is related to friction.
The majority of the time, as the clipboard was raised by the clip, 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. For example, in 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 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, on 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.
For Further Exploration
This science activity can be expanded or modified in a number of ways. Here are a few options:
- You could repeat this activity but use a protractor to quantify your results. What is the exact angle of repose, or angle at which the different penny stacks start to slide down the slope? How do their angles of repose compare exactly?
- How does the size of the penny stack affect how it slides down the slope? You can try this activity again by making different-sized penny stacks (with more or fewer pennies) and compare their angle of repose.
- You could try this activity with other objects and see if you get similar results. For example, you could make different objects from LEGO® bricks and repeat this activity.
CreditsTeisha Rowland, PhD, Science Buddies
Sponsored by a generous grant from Chevron