Student Guide: Landslides: What Causes Rocks to Slip and Slide?

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Summary

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 model landslides and investigate how friction and the angle of a hill's slope affect them.

Useful Vocabulary

• Landslide: Any geological process resulting in a downward movement of material from a slope of rock, soil, artificial fill, or a combination of the three, under the influence of gravity.
• Gravity: The force that pulls objects down, toward the center of the Earth. This force is related to an object's mass. When an object is on a slope, the effect of gravity can be separated into a part that is parallel to the slope (pulling the object down the slope) and a part that is perpendicular to the slope (pulling the object against the slope's surface). As the angle of the slope increases, the parallel part of gravity increases, and the perpendicular part decreases.
• Friction: Resistance to movement when two objects, or surfaces, rub against each other. The amount of friction depends upon the part of gravity that is perpendicular to the slope's surface.
• Angle of the slope: How tilted a slope is compared to a flat, horizontal surface. Angles are measured in degrees.
• Angle of repose: The angle at which the force of gravity will overcome friction and cause an object to start sliding down a slope.
 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.

Materials

To do this activity, you will need:

• Stack of four pennies taped together (2). One stack should have a paper towel strip on the bottom of it.
• Clipboard, hard (1)
• Paper towel (1 sheet)

Directions

1. Set the clipboard on a flat surface. Clip the paper towel sheet on the clipboard. (If there is a small strip cut from the sheet, put it at the bottom.)
2. Place one of the penny stacks on the paper towel-covered clipboard, touching the clip at the top. Then place the other penny stack on the clipboard, next to the first stack, also touching the clip.
3. Make sure both stacks are placed so that their rough tape edges are facing up (with the paper towel strip and smooth-taped side of the stacks facing down, touching the paper towel-covered clipboard). How does the bottom of each stack feel compared to the other? Is one much smoother than the other?
 Figure 2. Clip a paper towel to the clipboard and then place the two stacks of pennies on the paper towel, touching the clip.
1. Holding on to the clip, slowly and steadily lift the clipboard, making sure to lift only the side where the clip is. Which stack of pennies slides down the clipboard first? Stop tilting the clipboard as soon as one of the stacks of pennies starts to slide down.
 Figure 3. Slowly lift the clipboard, tilting it up from the clip, and watch to see which group of pennies slides down first.
1. Repeat this process at least nine more times, for a total of ten trials. Each time be sure to start with the clipboard lying flat on a flat surface, 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 slid down the clipboard first? Are your results fairly similar?
2. 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 may have been different for the two different stacks of pennies? What do you think your results might have to do with friction?