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Buoyancy Forces

Buoyancy Forces. Simply put pressure. When an object is floating on top of a liquid the object’s bottom experiences a forces from the pressure exerted by the liquid while the top does not. F Buoyancy. Pressure. This forces is the buoyancy force. F g.

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Buoyancy Forces

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  1. Buoyancy Forces

  2. Simply put pressure. When an object is floating on top of a liquid the object’s bottom experiences a forces from the pressure exerted by the liquid while the top does not. FBuoyancy Pressure This forces is the buoyancy force. Fg If the object is floating /FBuoyancy/ must equal /Fg/ Why do objects float?

  3. htop Pressure Head hbottom Dh Pressure Resultant = (rlig)(ag)(Dh) Do immersed objects feel a buoyancy force? • Because the pressure a fluid exerts depends on it’s depth, the object’s bottom will always experiences more pressure than it’s top. • This results in a net pressure (and net force) upwards. • This net force (from the top and bottom pressures only) in the buoyancy force.

  4. FBuoyancy Fg Do immersed objects feel a buoyancy force? Dh PressureResultant = (rlig)(ag)(Dh) However, in this case, the /FBuoyancy/ is less than /Fg/. The resultant of these two opposing forces is called the apparent weight. Apparent weight is simply how heavy something appears to be will in given liquid. Apparent Weight = /FB/ - /Fg/

  5. FBuoyancy Fg Calculating Buoyancy force Pressure Force = (Pressure)(Area) FB = (Pressure on bottom)(AreaBottom) FB = (rlig)(ag)(Depth)(AreaBottom) (Depth)(AreaBottom)= VolumeSubmerged FB = (rlig)(ag)[(Depth)(AreaBottom)] FB = (rlig)(ag)[(Vol.Sub)]

  6. [ ] Vol.Sub Vol.Sub < Vol.object robject= rliq Vol.object robject rliq < For an object to float in a liquid the object’s density must be less than the liquid’s. Which objects float? To float: /FB/ = /Fg/ (rlig)(ag)[(Vol.Sub)]= (mobject)(ag) (mobject) = (robject)[(Vol.object)] (rlig)[(Vol.Sub)]= (robject)[(Vol.object)]

  7. /FB/ = /Fg/ (rlig)(ag)[(Vol.Sub)]= (mobject)(ag) (rlig)[(Vol.Sub)]= (mliq) (mliq)= (mobject) Archimedes principle • For an object to float in a fluid. The object must displace its own weight (or mass) worth of that fluid. • A 100 pound object floating in water must be able to displace 100 pounds of water. An egg sinks in fresh water, but floats in salt water. Why?

  8. Specific Gravity • An object’s Specific Gravity (S.G.) is a ratio of the object’s density to that of fresh water • rfresh water = 1 g/cm3 = 1000 kg/m3 • S.G. = (Density of object)/(Density of Fresh water) • Specific gravity has no units. Why?

  9. The required submerged volume for an object to float = (S.G.)(Volume of object) Specific gravity and submerged volume (Fresh water systems only) [ ] Note: only objects with a specific gravity that is less than 1 can float in fresh water. If the S.G. is greater than 1 the required submerged volume is greater than actual volume of the object, so it must sink.

  10. Apparent mass • When a object is placed in a fluid it appears to be “lighter” than it “really is” because of the buoyancy force. • Since these object’s appear lighter, may people say these objects appear to be less massive. • Is this good terminology of bad? Why? • The measured mass of an objected when it is in a fluid is called the apparent mass • It is the mass the object appears to have when it the fluid.

  11. Fg Apparent ---- g Fg ---- g mapparent = m = FBuoyancy FgApparent = Fg - FBuoyancy Fg – Fbuoyancy ------------- g mapparent = Fg Calculating Apparent mass

  12. Fg - FBuoyancy ---------------- g mapparent = (m object)g - (rLiq)(g)(Volsubmerged) -------------------------------- g mapparent = m apparent = (m object)- (rLiq)(Volsubmerged) If the object floats it has an apparent mass of 0 kg (a scale will not be able to “measure the object’s mass”).

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