Chapter 13: Unsafe Ground: Landslides and Other Mass Movements
Feature Articles
Energy and Work During Mass Movements
by Stephen Marshak
Thustle, UT landslide
Credit: USGS
To understand what happens during downslope mass movement, we need to consider a few basic concepts from physics, namely potential energy, kinetic energy, force, and work. To simplify our discussion, let's look at a simple example: a rock sitting at the top of a mountain moves to the bottom of the mountain.
Any object in a gravitational field feels an attractive force. Earth's gravity wants to pull objects closer to the center of the planet, so it tries to pull the rock down to a lower elevation. While the rock sits at the top of the mountain, therefore, it stores gravitation potential energy—it has the potential to fall down, if given the chance. If the rock starts to tumble down, most of this potential energy transforms into kinetic energy, the energy of motion, and the rest transforms into heat, generated by friction.
Once a mass starts moving, it has momentum. Physicists define momentum as the product of mass time velocity (p=mxv), so the greater the mass, the greater its momentum at a given velocity, and the greater the velocity, the greater its momentum for a given mass. When the moving rock strikes other rocks in its path, it transfers art of its momentum to the other rocks and sets them in motion-these rocks, in turn, can set other rocks in motion. You see this phenomenon happen when a moving billiard ball strikes another. Thus, a single rock falling down a cliff can trigger a large avalanche.
We can also look at a mass movement in terms of the "Work" involved. Physicists define work as the product of force time displacement (the change in the position of the mass); in equation form, W=FxDd. Remember that force (F) equals mass times acceleration, so the application of force to an object can change its velocity. Geological forces (such as may be caused by a continent-continent collision) do work when they lift rocks to form a mountain range. People do work when they lift debris and dump it in a pile. When a rock falls from the top of a mountain or debris pile down to the adjacent valley floor, gravity does the work. The amount of work done by gravity in moving the rock down equals the amount done by geologic or human forces in lifting the rock up.
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