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Main Ideas n n n n n n Forces Within Solids Forces within solids Pressure Buoyant forces: Archimedes Principle Floating objects Forces within the earth’s crust: isostasy Convection Chunk inside wall Building Contact Forces Weight Pieces of solid objects must have internal contact forces equal to their weight. what forces act on the block? n n n significant gravitational force? charges involved? is anything touching the object (contact forces)? a. normal force b. friction what is the motion of the block? acceleration = 0 net force = 0 n n Frightened Fish What laws and principles apply? Pressure Shark Fish the contact forces and gravity act equally on the block n n n n Fish contact force acceleration n Pressure=Force/Area Pressure of a static fluid is perpendicular to surfaces immersed in the fluid Pressure of fluid is the same in all directions Pressure increases with depth Pressure is the same for all points at the same depth weight PS100, Section 3, Winter 2000 Chapter 6, Page 1 Atmospheric and Hydrostatic Pressure n Atmospheric pressure at the earth’s surface is about 15 psi. Buoyant Forces n n Fluids always push up on anything they surround. Two ways of thinking about it: 1. n Water pressure is about 1 atmosphere every 11 meters. 2. United States Department of the Interior Bureau of Reclamation - Lower Colorado Region Archimedes Principle n Pressure is greater on the bottom Fluids can support their own weight. The upward force is equal to the weight of the displaced fluid. Immersed Object An object immersed in a fluid experiences an upward buoyant force due to contact interactions with the surrounding fluid, whose strength is equal to the weight of the displaced fluid. Contact Forces Gravity Three Immersed Balls Gravity Floating Objects buoyant force lead water cork n weight PS100, Section 3, Winter 2000 According to Archimedes Principle, an object will sink until its weight equals the weight of displaced fluid. Chapter 6, Page 2 Brain twister n Iceberg Example So, how much water does it take to float a battle ship? a. A weight of water equal to the weight of the ship. b. A volume of water equal to the volume of the ship. c. A few cups. Forces Within the Earth’s Crust Convection n n n n n n Isostasy-balance between the weight of the earth’s crust and buoyancy of the mantle crust floats on mantle taller features (mountains) have deeper roots Ways to say something floats n n n n cold (high density) fluids sink warm (low density) fluids rise this creates circulating currents It is less dense than the surrounding fluid. It weighs less than an equal volume of fluid. The buoyant force is equal to its weight when it is only partially submerged. It displaces a weight of fluid equal to its weight when it is only partially submerged. STOP! n Remember to read Chapter 9 before next class. You must compare weight and buoyant force. PS100, Section 3, Winter 2000 Chapter 6, Page 3