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Introduction to Physical Science Monday, Wednesday, Thursday Tom Burbine tomburbine@astro.umass.edu

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Density Density = Mass/Volume Mass is usually in grams or kilograms Volume is usually in cm³ or m³

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Densities Density of water = 1 g/cm³ = 1,000 kg/m³ Density of iron = ~8 g/cm³ = ~8,000 kg/m³

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Weight density Weight density = weight/volume

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Pressure Pressure = Force/Area Unit of pressure is a Pascal, –which is a newton per square meter

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Pressure in a liquid When you swim, you feel the pressure of the water plus air above you It is the weight of the fluids (water and air) above you Swim deeper, more pressure

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Pressure in a Liquid Liquid pressure = weight density x depth

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Pressure in a liquid Liquid pressure is exerted equally in all directions

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Buoyant Force Upward acting force, caused by fluid pressure, that opposes an object's weight

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Equal Volumes Equal volumes feel equal buoyant forces Cork would float, aluminum would sink, and lead would sink faster Difference in their behavior is due to their weight

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Archimedes (287 BC-212 BC) Greatest scientist of antiquity Needed to determine if silver had been substituted for gold in a crown for a king Mass could easily be determined but not mass Discovered that you could determine the volume of the crown by submerging it in water Density (mass/volume) could be determined and compared to pure gold This story may not be true

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Archimedes Principle An immersed body is buoyed up by a force equal to the weight of the fluid it displaces

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Meaning When an object is immersed in water, it feels lighter Apparent weight of object is the weight of object minus the buoyant force N 3 N of N

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Principle of Flotation A floating object displaces a weight of fluid equal to its own weight

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Why does an iron ship float? Iron is about eight times as dense as water When submerged, displaces 1/8 of a ton of water If the iron is in a bowl and displaces 1 ton of water, it floats

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Buoyant force equals the weight of the fluid it displaces Denser fluids exert more buoyant force upon a body than less-dense fluids of the same volume A ship floats higher in salt water than in fresh water because salt water is slightly denser than fresh water so it has higher mass and exerts a higher buoyant force

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http://www.youtube.com/watch?v=eQsmq3Hu9H Ahttp://www.youtube.com/watch?v=eQsmq3Hu9H A

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Any Questions?

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