Transcript Homework:
THE BUOYANT FORCE Fbuoy HOW BIG IS THE BUOYANT FORCE? Fbuoy 1m 1m 1m THE CASPER BOX Fbuoy = 10,000 N - no mass - just holds stuff 1m WATER How big is the buoyant force? WE,W = 10,000 N 1m 1m THE CASPER BOX Fbuoy = 10,000 N - no mass - just holds stuff a 1m STEEL How big is the buoyant force? WE,S = 78,000 N 1m 1m THE CASPER BOX Fbuoy = 10,000 N - no mass - just holds stuff a 1m How big is the buoyant force? STYROFOAM WE,S = 1,000 N 1m 1m Archimedes’ Principle: • The buoyant force acting on an object is equal to the weight of the fluid • displaced by the object, or • that would fill the “Casper Box” • If an object water it will • If an object water it will has greater density than sink has smaller density than float Where will they “float?” oil r = 0.9 g/ml fresh water r = 1.0 g/ml salt water r = 1.2 g/ml Object A: rA = 0.8 g/ml Object B: rB = 0.95g/ml Object C: rC = 1.1 g/ml Object D: rD = 1.3 g/ml Object E: rE = 4.0 g/ml How can a steel boat float? The bouyant force is greatest on a ten ton ship if it is floating in: • A) fresh water • B) salt water a=? • C) mercury F(buoy)W,b a=0 WE,b • D) all the same What does change? THREE BLOCKS • Which block has the greatest buoyant force? Which block experiences the largest buoyant force? Archimedes’ Principle Fw,1 Fw,2 balsa oak WE,2 WE,1 What should I remember? • Archimedes’ Principle always applies, whether an object sinks or floats. • buoyant force is always equal to weight of fluid displaced. • If an object floats, the buoyant force is also equal to the object’s weight. Problem • A cube of wood 20 cm on each side is placed in a tub of water and is found to sink so that only 6 cm of the block is above the water. What is the density of the wood? 6 cm 20 cm Hard problem: You are floating in a boat in your very own swimming pool (you are rich). There are six gold bricks sitting on the floor of the boat (you are rich). You throw them into the pool. Does the water level of the pool go up or down (or stay the same)?