# Objects have a gravitational attraction to each other

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objects have a gravitational attraction to each other and the strength of the attraction is based on mass of the objects and their distances. The earth has much mass, so it attracts everything near it. This gravitational force pulls everything toward Earth's center. For this reason, water flows downhill (if it can't fall straight down, it will flow downhill), and the same for glaciers. Solar energy also affects erosion, because it drives Earth’s weather. Let’s go back to some simple physics of relevance to geomorphology. If an object is moving, it has kinetic energy , and if the object encounters another object, some of that kinetic energy is used– force is applied to the other object. If an object is located above its surroundings, it is said to have potential energy . This means that because of its position, gravity has the potential to pull it down and convert the potential energy to kinetic energy. A rock sitting on a hillside has potential energy because of its position. If you push it and start it rolling, the potential energy is converted to kinetic energy, and it can exert force on anything with which it comes in contact. Any object above base level, then, has potential energy and a peneplain essentially has no potential energy. As to resistance , when a force is exerted on an object, it will move only if the force exceeds the object's resistance to that force. An object's mass is important here: it takes more force to move a heavy object than a light one, so the heavy one has a higher resistance. Weathering, by breaking rocks into smaller pieces, makes it easier for material to be eroded. Different types of rocks have different resistances to weathering and erosion. The arrangement of rocks, or structure , affects their resistance to erosion. If relatively resistant rocks lie on top of weak rocks, the weak ones are protected. Both forces and resistances vary over the earth's surface. Where erosional force exceeds resistance, erosion will occur and the earth's surface will be
GEOG 1401 Physical Geography 19—151 Ó Jeffrey A. Lee lowered. The surface will be lowered most where the force most exceeds the resistance. Where resistance is greater than force, the earth's surface will remain relatively unchanged. We view the landscape as a balance between forces and resistances , with the surface tending toward an equilibrium state where the two are balanced. This equilibrium may never be achieved because of the fluctuations of the forces, so steady state or dynamic equilibria may exist. If the forces vary greatly, the system may never even approximate an equilibrium situation. An important consideration in this line of inquiry is the amount of time a landscape has had to achieve such an equilibrium condition. After a volcano changes an area, it takes time for the land to adjust to the new conditions. Depending on the situation, some landforms adjust in a matter of minutes (wind ripples), some years (river meanders), or millions of years (mountain ranges).