Buoyancy (2).pptx

How do objects float. Why doesnt a steel ship sink if steel is denser than water. Why do some rocks float

1. Buoyancy

2. Buoyancy Upward net force on objects within a liquid Occurs because pressure in a fluid increases with depth Since the bottom is deeper, more force is pushing up than force on the bottom pushing down Net upward force pushes objects up

3. Archimedes Principle The buoyant force on an object is equal to the weight of the fluid displaced by the object F B = F w = mg ρ = M/V M = ρ V F B = ρ Vg It is NOT the object that exerts the buoyant force, it is the fluid that the object displaces that causes it F B = ρ fluid gV submerged

4. The difference between the buoyant force and the object’s weight determines whether an object sinks or floats. Sink or Float

5. What is the buoyant force on this object?

6. Example A 70kg statue lies at the bottom of the sea. Its volume is 3.0 x 10 -2 m 3 . How much force is needed to lift it? When a crown of mass 14.7kg is submerged in water, an accurate scale reads only 13.4kg. Is the crown gold?

7. Floating An object floats on a fluid if its density is less than that of the fluid When floating F B = F W ρ f V disp g = ρ o V o g ρ f V disp = ρ o V o Fraction of an object submerged is given by the ratio of the density of the object to the fluid

8. Example A rectangular block of wood (1.0m x 2.0m x 0.25m) floats so that 0.15 cm of its height is above the water level. a) find the density of the wood b) find the mass of the wood