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Sliding Science: How Are Landslides Caused?

A physics problem from Science Buddies


Key concepts
Slopes
Angles
Forces
Gravity
Friction

Introduction
Have you ever seen a video of a landslide? Landslides are powerful geologic events that happen suddenly and cause devastation in areas with unstable hills, slopes and cliff sides. Each year in the U.S. landslides can cause great damage to buildings and property, in addition to changing 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.

Background
A landslide is any geologic process in which gravity causes rock, soil, artificial fill or a combination of the three to move down a slope. Several things can trigger landslides, including the slow weathering of rocks as well as soil erosion, earthquakes and volcanic activity.

One major force all landslides have in common is that they are propelled by gravity. We normally think of gravity pulling an object vertically down, such as when you drop a ball straight down. But on a slope gravity gets slightly more complicated. Any force (such as gravity) has magnitude and direction. On a slope gravitational effects can be separated into a component that's parallel to the slope (which pulls the object down the slope) and a component perpendicular to it (which pulls the object against the slope's surface). As the angle of the slope increases (making it steeper), gravity's parallel component increases and the perpendicular component decreases, thereby overcoming resistance for downward movement. This resistance is called friction and depends on the perpendicular component of gravity, along with the slope's and object's surfaces. When the parallel component becomes greater than the perpendicular component, the object slides down the slope. In other words, the critical maximum slope from horizontal—called the angle of repose, which is the greatest angle that an object will remain at rest—has been surpassed.

Materials
Clear tape
Eight pennies
Hard clipboard
Paper towel
Ruler (optional)

Preparation
Take a piece of tape a little longer than the length of four pennies lined up next to one another (about three and a half inches long) and set two pennies on the tape so the pennies are touching, side by side.

Set one penny on each of the two pennies on the tape so that you have two stacks of pennies with two pennies in each stack. Then wrap the tape lengthwise 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 the 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 two and a half 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 lengthwise 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 only by the paper towel strip.

Procedure
Set the clipboard on a flat surface. Clip a paper towel sheet onto the clipboard. (If you cut a strip out of the sheet for the penny wrapping, put that space at the bottom of the clipboard.)

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 lengthwise but not touching one another.

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 with one another? Is one much smoother than the other?

Holding on to the clipboard 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 slides 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.

Repeat this process at least nine more times for a total of 10 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 one another by the clip. Also make sure to slowly lift the clipboard each time. For each trial, which stack of pennies slides 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 after 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 size 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? What factors do you think are most important in determining whether an object will first slide at a lower or steeper incline

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