TeachingElementaryScience

Does it Sink or Float?

What allows something to sink or float?  An object will sink if it is heavier than water, right?  Let’s explore that concept a bit more.   Explain how a heavy steel ship floats in the ocean.  Surely it is heavier than water.  Well perhaps there is something else happening.  Did you ever notice that when something floats in water, part of it is actually under water? As it sinks (even a little bit) it pushes away the water until that amount of water weighs the same as the object that is floating. If the object you are trying to float is too heavy, it cannot push away enough water to be the same as how much it weighs. If that happens, the object will sink.

But wait; there is more to this concept of sink or float. Density is another important concept associated with sink or float.  This property called density, which is the mass of an object divided by its volume D = M/V, effects sinking and floating.  Here is how it works.  Take an empty water bottle.  Will it sink or float?  Yes of course it will float because its mass is less than the water.  Now take the same water bottle and fill it with sand.  Will it sink or float?  Yes of course it will sink because its mass is more than water.  Let’s explore the concept of density a bit more.  Things that have a lower density than water will float in water. This is because the item weighs less than the water that it displaces.

Hold on there is another factor that influences sink or float that we need to explore.  I am sure you have heard of buoyancy as it relates to the concept of sink or float.  Try this experiment.  Take a piece of aluminum foil and making a water-tight boat out of it. If you carefully put the boat in a dish or pan of water, you will see that it floats.  Now take the aluminum foil boat and crumple it up into a ball and put it back on the water. It sinks! There is the same amount of aluminum foil in both cases, but in the case of the boat, you shaped it so that it displaced a lot of water compared to the amount of water that is displaced when you crumpled the aluminum foil into a ball.

Here is a great youtube book reading of Who Sank the Boat? by Pamela Allen:

http://www.youtube.com/watch?v=OsYb1YSYR34

Here is a video of a group of 3rd – 5th grade teachers experimenting with physics through games. Watch the video and see if you can explain what forces are being used through the games. A minute to Win it – A discovery in force and motion