Chapter 11 – Forces in Fluids Section 2 – Floating and Sinking Standards 8.c – Students know that buoyant force on an object in a fluid is an upward force equal to the weight of the fluid the object has displaced 8.d – Students know how to predict whether an object will float or sink

Density Density – mass per unit volume Density = mass / volume (g/ml or g/cm3) The density of water is 1.0 g/ml or 1.0 g/cm3

Density An object that is more dense than the fluid in which it is immersed sinks An object that is less dense than the fluid in which it is immersed floats

Density Practice A sample of liquid has a mass of 24 g and a volume of 16 mL. What is its density? Does the liquid sink or float? Density = mass / volume Denisty = 24 g / 16 mL Density = 1.5 g/mL Denisty is greater than 1.0 g/mL so it sinks

Calculating Density Practice Problem A piece of metal has a mass of 43.5 g and a volume of 15 cm3. What is its density? Density = mass / volume  Density = 43.5 g / 15 cm3 Denisity = 2.9 g/ cm3  

Densities of Substances By comparing densities, you can predict whether an object will sink in a fluid 0.70 g/cm3 = wood 0.92 g/cm3 = corn oil 0.93 g/cm3 = plastic 1.00 g/cm3 = water 1.02 g/cm3 = tar 1.26 g/cm3 = glycerin 1.34 g/cm3 = rubber 1.38 g/cm3 = corn syrup 8.80 g/cm3 = copper 13.6 g/cm3 = mercury

Density Changes in density cause a submarine to dive, rise, or float.

Density Changes in density cause a submarine to dive, rise, or float.

Density Changes in density cause a submarine to dive, rise, or float.

Buoyancy The buoyant force acts in the direction opposite to the force of gravity, so it makes an object feel lighter Buoyant Force – upward force exerted by a fluid on a submerged object

Buoyancy The greater pressure at greater depths means the net force is greatest pushing upward on a submerged object

Buoyancy Archimedes’ principle states that the buoyant force acting on a submerged object is equal to the weight of the fluid the object displaces

Floating and Sinking If the weight of the object is greater than the buoyant force – then the net force will be downward and the object will sink If the weight of the object is less than the buoyant force then the net force will be upward and the object will float

Buoyancy and Density An object with greater volume has a greater buoyant force acting on it – even if the object is more dense than the fluid it is immersed in Density of iron lead is 11.3 g/ml and cork is 0.25 g/ml – lead sinks, cork floats