Chapter 7 Section 2

Explain the relationship between fluid pressure and buoyant force

Predict whether an object will float or sink in a fluid

Analyze the role of density in an object’s ability to float

Explain how the overall density of an object can be changed

The upward force that keeps an object immersed in or floating on a liquid

Water exerts pressure on all sides of an object

Horizontal pressures cancel each other out

Top an bottom pressure affect net force

The pressure at the bottom of the object is greater than the pressure at the top; upward net force (buoyant force)

Archimedes Principle: the buoyant force on an object in a fluid is an upward force equal to the weight of the volume of fluid that the object displaces

Ex: An object that displaces 250mL of water, the weight of that volume of displaced water = 2.5N, therefore the buoyant force is 2.5N

(1L of water weighs 10N)

Sinking:◦ A rock 75N, displaces 5L of water◦ Weight of displaced water 50N, object sinks

Floating:◦ A fish 12N, displaces 12N of water◦ Fish weight = displaced water (suspended)

Buoying Up:◦ A duck partially submerged displaces 9N of water◦ When buoyant force > object’s weight

A force of 15N is required to lift an object that is underwater. The object displaces 2L of water.

1L of water weighs 10N

What is the weight of the object out of water?

Equation: ◦ Force required to lift object in water =

weight of object out of water – buoyant force

• 15N = weight of object out of water – 20N

• Weight of object out of water = 20N + 15N = 35N

The rock sinks because it is more dense than water

The duck floats because it is less dense The fish is suspended because it is = to the

density of water

Density of water= 1g/mL Density = mass/volume 1mL = 1cm³

More dense than air:◦ Most substances are more dense than air◦ Ice cube floats in water, not in air

Less dense than air:◦ Helium is less dense than air

Density = mass ÷ volume

Mass = density x volume

Volume = mass ÷ density

Page 188 in text: Practice Problems

Changing shape:◦ Steel ships are built with a hollow shape to increase the

volume, decreasing density

Changing mass:◦ Ballast tanks are built into submarines to increase mass

while maintaining volume, density increases◦ Allows submarines to dive deep in the ocean

Changing volume:◦ Fish use swim bladders to increase volume, decreasing

density

How can you determine the buoyant force acting on an object?

Who discovered how to determine buoyant force?

How can a scuba diver keep floating back to the surface of the water?