Will a bowling ball sink or float?
Needed:
· 6 lb bowling ball
· 12 lb bowling ball
· 6 lb exercise ball
· Tub of water
Teaching Points:
1) The main teaching point for this activity is about density. (Density = mass / volume) We would like the visitors to grapple with the idea of density (which is different than weight or mass.)
2) Saturn is the only planet in our solar system that is less dense than water. (It is about 70% as dense as water, and so would float in a giant tub of water with about 70% of Saturn below the water and 30% above.)
Activity Outline:
1. Hold up 6 lb bowling ball and ask who thinks it will sink or float. (This is a good place to get more visitors involved because you can go around and ask people individually, including people at the periphery.) (It floats.)
2. Hold up 12 lb bowling ball and ask who thinks it will sink or float. (It sinks.) Have visitors suggest why the 6 lb one floats and the 12 lb one sinks.
3. Hold up the 6 lb exercise ball and point out that it is the same weight as the 6 lb bowling ball which floats. Ask who thinks it will sink or float. (It sinks.)
4. Why does the bowling ball float but the exercise ball sink? The same mass (or weight) is compacted into a smaller volume in the exercise ball, so the exercise ball is more dense. It is denser than the water.
5. Explain that Saturn is the only planet in the Solar System that would float in water. However, it floats about 70% of the way into the water. Demonstrate this by pushing the 6 lb ball down so it is 70% of the way in.
More about density and why things float or sink.
(Nice pictures, huh?)
1. In figure 1, the 6 lb ball is floating in the water. That means that the rest of the water has enough lifting power to hold the ball up. This is denoted by the arrows.
2. In figure 2, you can see that the truth of the matter is that those “arrows” could hold up 6 lbs of anything. It could hold up a 6 lb ball OR 6 lbs of water. Since 6 lbs of water would just fit into the “hole”, you can see that the amount of water that the bowling ball displaces when it is floating must be 6 lbs of water.
3. In other words, an object floats at the point that it displaces its own weight of water.
4. If the object is too dense, like the 6 lb exercise ball is, than as you lower it into the water, it is physically too small to displace enough water to match its weight, so it sinks. The rest of the water does not have enough lifting power.
With the three balls listed above, you have two that are the same size, but different masses, and you have two that are the same mass, but different sizes. Use these to help visitors get their minds around the idea of density.
By the way, the thing that makes a Black Hole a Black Hole is not its mass, it is it’s density. Black Holes are extremely dense. The material in them is compacted to the extreme.