# Equation for density and why things sink or float. Part 1

## Floating and SinkingObjects

Submitted by: Matt K
Last date modified: 9/27/11

# Floating Objects:

A tree is very big, and very heavy, but its molecules are very spread apart, since the molecules are grown apart the tree will start floating. It will only sink deep enough to displace a volume of fluid with a weight equal to its own. At that level, it will stop sinking deeper, and will float. If the weight of the object is exactly equal to the buoyant force, the two forces are balance. But it is not just the weight that matters, it’s also the molecules, if the molecules are more spread apart in the object, the object will float. A tree is very big, and very heavy, but its molecules are very spread apart, since the molecules are grown apart the tree will start floating. Density=Mass/Volume If you put a Rubber ducky in a bathtub, is because it is very light, and because all of its particles (molecules) are very spread apart.

## This penny is very small and light, but its molecules are packed together making it float.

Sinking Objects:
There is always a downward force on a submerged object
. The object will always sink if there is a downward force on the submerged object. The force is the weight of the object. The net force on a submerged object will always be downward For example: A penny is very small and very light, but the particles (molecules) are very packed together, if you flip a penny into a fountain, you can see a penny will sink. If the molecules are more packed together, it will add on to the weight therefore the object will sink. If the Molecules are very tightly packed, the object will still sink. This is because all of the molecules are tightly packed together, and they were all combined into one heavy mass, and then it adds the objects weight which makes it heavier. The water tries to make room for the object that is how the water splashes when you throw an object from a distance. But even if you take the smallest object in the world, and the molecules are tightly packed together, the object will sink.

# Molecules

It’s not just the weight or mass that matters if an object floats or sinks. It also matters whether or not the molecules are spread apart, or tightly packed together. If the molecules in an object are tightly packed together it forms its own mass, and that mass gains on to the original mass of the object. Also if the molecules are spread far apart, it does not form any mass at all thus making the objects mass much lighter. For example: In our bodies we have a lot of molecules, and they are very tightly packed together, but some are still spread apart. That is why we float halfway when we are in water. Sometimes we float, but sometimes we sink, that’s because of our molecular motions, if we calm our bodies and don’t do anything at all, our molecules will stay in place and we will begin to float. But if we spaz out and move our arms, we begin to sink. For example, this rubber ducky is very light because there is nothing inside of it, but not only that the ducky has molecules that are very spread apart, that makes the rubber ducky very light, plus because it has nothing inside of it. (Molecules)

# Density

Why do some objects float and others sink? By comparing the density of an object to the density of a fluid, you can decide if it will float. The density of a substance is its mass per unit volume. For example, one cubic centimeter of lead has a mass of 11.3 grams, so its density is 11.3 g/cm3. In contrast, one cubic centimeter of cork has a mass of only about 0.25 gram. So its density is about 0.25 g/cm3. You would say that lead is denser than cork. The density of water is 1.0 g/cm3, so it is less dense than lead but more dense than cork. An object that is denser than the fluid in which it is immersed sink. An object that is less dense than the fluid in which it is immersed floats to the surface.

Citations:
Kahan, Peter Science Explorers-Forces, Energy, Motion Needham, Mass 2000
Discovery Streaming -The Buoyancy Debate: What Makes Objects Sink or Float?. Prod. Louisiana Public Broadcasting. Louisiana Public Broadcasting, 1995. Discovery Education. Web. 22 September 2011. http://www.discoveryeducation.com/
Discovery Streaming- Explaining Buoyancy: Buoyant Force and Floating Objects. Prod. Exploration Production Inc.. Exploration Production Inc., 2005. Discovery Education. Web. 22 September 2011. http://www.discoveryeducation.com/