1. 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

2. 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

3. 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

4. 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

5. 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  

6. 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

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

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

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

10. 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

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

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

13. 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

14. 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