The Physics of Floating Objects
The wood pictured below is solid throughout and is floating in water.
Let’s start by labeling our forces on the object. Are the forces balanced?
In the case of a floating object:
But we can also collect the water displaced…
What determines how high the object floats in the water?
Physics notes over Floating Objects, p. 1
When will objects go from floating
above water to floating in the
water?
If water’s density is 1000
, then how do we make boats out of
iron that float when iron’s density is 7900
? How can we take a
material that should sink and make it float?
A boat can float as long as it can ______________________________.
(If this bothers you, consider this: The average density of the hull – the
volume/mass of the iron plus the volume/mass of air below the water level – is less
than that of water. This density is the EFFECTIVE density of the boat).
Physics notes over Floating Objects, p. 2
We can come up with a general statement of floating:
This interesting ratio tells us the % of the object’s volume below the
fluid’s line (i.e. water line). For example, an
iceberg floating in the ocean:
This formula can be further reduced if
our object is a rectangular prism.
Physics notes over Floating Objects, p. 3
So if we have a wooden block (
immersed in water, how much of it
will be submerged (under the water line)?
We can come up with a general statement of when an object will sink or
float when placed under the water and then released:
So if an object is submerged and released:
a) if
<
, then
b) if
>
, then
c) if
=
, then
Physics notes over Floating Objects, p. 4
Buoyancy Practice Problems
;
(1) An oak log (
the water line in a fresh water lake
is the water line in the ocean
;
) floating down a river. Where is
? Where
?
(2) From the online HW: A block of volume 0.59 m3 floats with a
fraction of 0.72 of its volume submerged in a liquid of density 1210
kg/m3 as shown in the figure below:
Find the magnitude of the buoyant force on the block. Correct answer =
5037.28
Physics notes over Floating Objects, p. 5