Worksheets and lesson ideas to challenge students aged 11 to 16 to think hard about floating, sinking and upthrust (GCSE and Key Stage 3)
Heavy objects sink, light objects float – right? You can have lots of fun teaching students about why objects float and the opportunity to address many of their prior conceptions. This demonstration below is a great way to set up some cognitive dissonance when students see that some heavier objects float! Click here for some other lovely demonstrations on floating and sinking and make sure you check out our page on density.
Does a cannon ball float?
Density is a good starting point to begin thinking about why some objects sink and some objects float. This great video, showing a cannon ball floating in a pool of mercury, can be used to help students understand the relationship between density of an object and the fluid and whether it floats or not. Depending on the age of the students, you could get them to calculate the density of the lead cannon ball and compare this to the density of water and mercury. Hopefully this will help students to understand why a cannon ball can float in one liquid but sink in another. You can then extend this idea to consider the concept of relative density (density of object/density of water) and see if students can identify how this concept can be used to predict whether an object will float or sink in a specific fluid i.e. >1 it will sink. This then begs the question – so how can a ship float?!
Weight and volume
This thought experiment requires students to understand that objects float in water when they displace a volume of water that is equal in weight to the weight of the floating object. The question really helps to see if students understand the concept of displacement. It goes like this:
A boat is on a pond and the anchor is thrown overboard. What will happen to the level of water in the pond?
- A. It will increase slightly
- B. It will decrease slightly
- C. It will stay the same
- D. It is impossible to tell
The answer is B. When an object is floating it will displace a volume of water that is equal to its weight. When an object is in water, it will only displace a volume of water that is equal to its volume. As the anchor is heavy, it will displace much more water when it is floating than when it is on the bottom of the bond – so floating pushes the water level up. This means that when the anchor is thrown overboard the level of water in the pond will decrease slightly. You can extend this idea by asking students what will happen to the level of water in a glass that contains a melting ice cube.