Buoyant Forces

Chapter 3 Section 2

Tell me why?

• Why does an ice cube float on the top of a glass of soda instead of sinking to the bottom of the glass?

• If you push the ice cube to the bottom of the glass why does it float back to the top?

Let me explain

• Ice cubes float because they are less dense than water in its liquid state.

• When you push the ice cube to the bottom of the glass there is a force that pushes the cube back up to the top of the glass.

• This force is called the buoyant force.

Buoyant Forces

• Buoyant force is the upward force that fluids exert on all matter

• Since water exerts fluid pressure on all sides of an object the horizontal pressures cancel each other out.

• Also since pressure increases with depth, the pressure at the bottom of an object will be greater than on top of the object. The net force is upwards causing the object to float.

Archimedes’ Principle

• The principle that states the buoyant force on an object in a fluid is an upward force equal to the weight of volume of fluid that the object displaces.

Sinking Vs. Floating

• An object in a fluid will sink if its weight is greater than the buoyant force (the weight of fluid the object displaces)

• An object floats only when the buoyant force on the object is greater than or equal to the object’s weight.

Sink or Swim?

• Make pictures

Density

• Density- the amount of mass per unit volume

• D=m/v

• Objects less dense than air will float while objects more dense than air sink (or rest on the ground)

• The same applies for objects in water

Density Triangle

m

D v

So if steel is 8x’s more dense than water, how do large ships float?

• Ships change their shape into a hallow form which increases the volume occupied by the same mass.

• The overall density of the ship is reduced.

• Since the ship is less dense than water, the ship floats!

Ships Changing Mass

• Some ships change their density by changing their mass over the same amount of volume. Submarines have ballast tanks with valves that allow water in or out.

• When the ballast tanks are filled with air, the sub is less dense than water and it floats.

• When the ballast tanks are filled with water, the sub is more dense than water and it sinks.

• Fish use a similar method to increase/decrease depth in water. (swim bladders instead of ballast tanks)