FORCES IN FLUIDSChapter 3

Section 2 - Floating and Sinking

Section 3 – Pascal’s Principle

Buoyancy

• Buoyant Force – upward force exerted by water and other fluids. It acts in the direction opposite of gravity.• Picking up objects underwater makes them seem lighter

• Fluid exerts pressure on all surfaces of an object under water.

Buoyant Force and Weight

• If an objects weight is greater than the buoyant force, the object will sink.

• If an objects weight is equal to the buoyant force, the object will not sink.

• If an objects weight is less than the buoyant force, the object will float.

• Figure 8, pg. 83

Archimedes’ Principle• All objects take up space• There is a connection between the weight of a fluid

displaced by an object and the buoyant force acting on it.• Archimedes’ Principle – the buoyant force acting on a

submerged object is equal to the weight of the fluid the object displaces.• Think of a full swimming pool – if it is filled to the top and you get in,

water will overflow making room for you in the pool. If you displace 50 L of water, the buoyant force on you will be equal to the weight of 50 L of water = 500N.

Density• Density = mass/volume• If an object is less dense than

water, it will float.• If an object is more dense

than water, it will sink• Density of water is about 1.0

g/cm3

• By comparing densities of objects and the liquids they will be going in, you can determine if they will sink or float.

Figure 11, p. 85

Changing Density• You can change the density of water by freezing it into ice.• Water expands when it freezes so ice takes up more

space than liquid water.• It is only a little less dense than water so most of it is

hidden below the water – like an iceberg

Figure 12

• You can make an object sink or float by changing density.• Ex. – Submarine

• Density will increase when the mass increases.

Figure 13, p. 87

• Air is also a fluid• If you fill a balloon with helium, it will float upward because

helium is less dense than air.

• A balloon filled with regular air will sink because it is more dense than air. (Air + balloon)

Pascal’s Principle• Pascal’s Principle states – when force is applied to a

confined fluid, the change in pressure is transmitted equally to all parts of the fluid.• In a closed container, when you press on one part of the bottle, the

pressure is felt throughout all the fluid within the bottle.• The water pressure increases everywhere.

Figure 14