This is a really simple demonstration that I use to illustrate that density differences result in objects floating. I mainly use it to setup a clicker question.
- A fish tank half full of water
- A variety of small balls such as ping-pong, squash, racquet, tennis, and golf. Make sure that some float and some do not. For the list given the golf ball sinks.
- Scale for weighing the balls and ruler for measuring their diameter (optional)
There are a number of ways to run the demonstration.
I simply place all the balls in the tank and observe which balls float and which do not. I then ask a follow up multiple choice question:
“why does the golf ball sink:
- It has dimples
- It is heavier than the other balls
- It is denser than the other balls
- It is harder than the other balls”
I use the IClicker system so I get instant feedback and almost all the students get this correct. This may not seem very profound but it does give them a visual reminder that less dense (lower specific weight) objects float and hopefully helps them recall that the buoyancy force is given by the volume displaced multiplied by the fluids specific weight (that is the weight of the fluid displaced).
Alternatively, you could predict which balls will float by calculating their density before placing them in the water using the scale and ruler. You can rank them by density (low to high) and then compare that ranking to the ranking of the fraction of the ball submerged.
Don’t forget to follow @nbkaye on twitter for updates to this blog. If you have a demonstration that you use in class that you would like to share on this blog please email me (email@example.com). I also welcome comments (through the comments section or via email) on improving the demonstrations.