Forces in Fluids

Buoyant Forces

Terms• Buoyant Force ~ the upward force

that fluids exert on all matter

• 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

Buoyant Force is Caused by Differences in Fluid Pressure

• Determining buoyant force– Based on the weight of the displaced

liquid– The weight of the object has nothing to

do with buoyant force– Ex… Block displaces - 250mL of water

250mL of water = 2.5N

Buoyant force = 2.5N

Weight vs. Buoyant Force• Sinking

– An object’s weight is greater than the weight of the water it displaces

– Ex. rock

• Floating– An objects weight is equal to the weight of

the water that it displaces– Ex. Fish, duck

• Buoying Up– When the buoyant force on the object

greater than the object’s weight…– Ex. Duck diving under and being pushed up

Sketch picture on bottom of page 169

An Object Will Float or Sink Based on Its Density

• More Dense Than Air– Ducks don’t float in air because they

are more dense than air– Most substances are more dense than

air

• Less Dense Than Air– Few substances are less dense than air– Ex. Helium

The Mystery of Floating Steel• Steel 8 times more dense than

water– Ships can float because of shape– Overall density less than water– Built to displace more water to

accommodate load

A Ship’s Shape Makes the Difference

The Mystery of Floating Steel• Density on the Move

– Submarines have tanks to take on water to increase mass

– An increase in mass increases density– Allows sub to sink (descend)– How does it rise (ascend)?

• How is a Fish Like a Submarine?